dormando edited this page Feb 27, 2018 · 22 revisions

External storage shim

Comprehensive documentation will be filled out over a couple days. If confused, come back in a bit :)

Requires memcached 1.5.4 or higher. Requires ./configure --enable-extstore during build time.

Feature for extending memcached's memory space onto flash (or similar) storage.

Quick Start

Extstore is an addition to memcached which leaves the hash table and keys in memory, but moves values to external storage (usually flash).

When to use this?

If you only have one or two memcached instances, you probably don't need this.

If stored values are much larger than keys and your hit rate would improve if you stored data for longer. You may save on cost (RAM can be expensive in "the cloud") if you have network capacity so spare.

There are useful tools online for analyzing memcached's stats output, but you can also simply telnet in and run stats and stats slabs and read the output. If you slab classes with large chunk_size values also have a relatively high number of total_pages, and you see evictions, you can give this a try.

You should also have flash storage. This will not work with rotational media.

How to use it?

Grab the latest memcached.

Build with: ./configure --enable-extstore && make && make test && sudo make install

Use your normal startup options (-m -c, etc), and add:

-o ext_path=/path/to/a/datafile,ext_page_count=100

Pages have a default size of 64 megabytes, so set this by how much disk storage you wish to use.

How to tune it?

Unless your servers are very busy, shouldn't do any further tuning. Try to monitor the output of stats and how much is being written to disk.

If you write too much to memcached, you may write too much to flash, which can burn out the drive or cause too much latency!

See below for a full description of tuning options.

Tuning guide

Before tuning, you should have a good grasp of your operating system, how to monitor latency, disk utilization, CPU utilization, etc. You should have a graphing system setup to monitor trends over time, including memcached statistics. You should be able to monitor your hit ratio, and graph various extstore_ stats via the stats command.

Why tune?

If you have a high capacity system (hundreds of gigs to terabytes), pushing hundreds of megabits to gigabits, you may see poor latency or poor hit rate. You may also see excessive writes to disk, which can age or burn out the device. Most server-quality flash devices are rated to be overwritten only 1-5+ times per day.

Primary tradeoffs

Items are flushed to flash storage from the tail of the LRU's, which are typically the least active items. The key and item structure (timestamps, flags, etc) plus 12 bytes for the flash location are kept in RAM. These are stored the same as normal items.

This means an item in slab class 50 using a hundred kilobytes of RAM would be flushed and reappear anywhere in slab class 1-6, depending on how long the key was.

If your values are only 60 bytes, you won't be saving much RAM (if any!).

Items are first written to a "write buffer" in RAM, which then gets flushed to a page. Once a page is full, it's capped and another one is then used.

Since items are pulled from the LRU tail, you are naturally less likely to hit the flash to retrieve them again later.

If you're writing new data to memcached faster than it can flush to flash, it will evict from the LRU tail rather than wait for the flash storage to flush. This is to prevent many failure scenarios as flash drives are prone to momentary hangs.

  • extstore is explicitly not a levelled storage system, which is often used to manage write loads. In order to support a cache workload on flash media, more data needs to be kept in RAM, and accesses to flash need to be direct to minimize latency.

Performance expectations

  • Many operations don't touch flash. IE: touch, delete, overwriting a value already on flash (including add), TTL expiration. They should be the same speed as without extstore.

  • Only reading items that were flushed, writing items from the end of the LRU, and compacting existing flushed items, touch storage.

  • In most deployments, items toward the end of the LRU are infrequently hit, but this can still be helpful if reducing misses has a large impact on hit ratio.

  • You may only want to flush very large items (64k+), which frees up the most RAM, and are very easy for a flash device to saturate a network with. The smaller the items stored on flash, the more likely you are to saturate the flash device before your network device.

What to watch for

  • evictions can mean you're out of RAM or you're writing too fast and extstore cannot keep up.

  • very low age (via stats items) in higher slab classes, may mean too much memory is in use for small items. You can use less flash storage, shorten keys, or add RAM to the server.

  • too much writing to disk. disk latency spikes. disk hangs. there aren't any internal measurements of disk latency as of this writing.

  • too much writing can be alleviated by adding more RAM and tuning compaction settings as per below. Recently set objects are the most likely to be re-written or deleted, so the longer they can be kept in RAM the less writing you end up doing on flash.

Page buckets

Storage "pages" are split into different "buckets", which allows extstore to colocate items. Currently there are buckets for:

  • Default, all non-special items end up here.

  • Large items (> 512k, or larger if the item size max is increased).

  • Compacted items (rewritten items are stored together to reduce fragmentation)

  • "low" ttl items. This bucket isn't used by default. Pages used by the low bucket are ignored by compaction, which can reduce writes to disk. By storing these separately from objects with a longer TTL, fragmentation can be reduced significantly.

Tuning options

(see --help or stats settings for the latest defaults. Version 1.5.4 may not contain all defaults in these outputs yet.

  • ext_page_size defaults to 64M. This shouldn't need to change unless you are storing very large values (multiple megabytes).

  • ext_wbuf_size defaults to 8M. One write buffer is needed per bucket, By default 32 megabytes of memory are reserved for write buffers. This cannot be larger than the page size. Larger write buffers can help with write rate performance.

  • ext_threads defaults to 1. If you have a high read latency but the drive is idle, you can increase this number. Stick to low values; no more than 8 threads.

  • ext_item_size defaults to 512 bytes. Items larger than this can be flushed. You can lower this value if you want to save a little extra RAM and your keys are short. You can also raise this value if you only wish to flush very large objects, which is a good place to start.

  • ext_low_ttl TTL's lower than this value will be colocated on disk. Off by default. Useful if you have a mix of high and low TTL's being used. Extstore has to compact or outright evict pages once flash is full. Mixing high and low TTL objects in a page will mean a page gets partially empty after some amount of time.

For example, if you only store objects with TTL's of 86400 (about a day) or 2400s. If you set ext_low_ttl to 2400 or higher, all of the latter objects will get stuck together. Once all of those objets have expired, the page they were in is reclaimed entirely. This avoids holes and compaction necessary if placed alongside the longer TTL'ed objects.

This option may change in the future; even more buckets may be useful.

  • ext_max_frag A compaction tuning flag. If there are few free pages remaining, pages which are at least this empty will be rewritten to reclaim space. IE: ext_max_frag=0.5 will rewrite pages which are at least half empty. If no pages are half empty, the oldest page will be evicted.

  • ext_drop_unread A compaction tuning flag. If enabled, and no pages match the ext_max_frag filter, a compaction will be done anyway. However, only items which are new or frequently accessed (ie; not in the COLD LRU) will be rewritten. This can cause more writing than simply evicting, but will keep active items around and raise hit ratio.

  • ext_compact_under A compaction tuning setting. If fewer than this amount of pages are free, check the ext_max_frag filter and start compacting. Defaults to 1/4th of the pages.

  • ext_drop_under A compaction tuning setting. If ext_drop_unread is enabled, and fewer than this many pages are free, compact and drop COLD data. This defaults to the same value as ext_compact_under.

  • ext_recache_rate If an item stored on flash has been accessed more than once in the last minute, it has a one in N chance of being recached into RAM and removed from flash. Defaults to 2000. It's good to keep this value high; recaches into RAM cause fragmentation on disk, and it's rare for objects in flash to become frequently accessed. If they do, they will eventually be recached.

Runtime tunables

Many of the values above can be adjusted at runtime.

Via a socket to memcached: extstore [command] [optional value]. As of this writing, commands are:

item_size item_age low_ttl recache_rate compact_under drop_under max_frag drop_unread

The effects are the same as documented above.

Future plans

TODO: Some info in the rough notes below, as well as doc/storage.txt.

Rough notes

See doc/storage.txt for API and some implementation details. See memcached -h for start arguments. Run ./configure --enable-extstore to compile the feature in.

Primary features:

  • very high speed. minimal CPU overhead.
  • writes are buffered and flushed sequentially. very good for improving read workload on most devices.
  • keys and headers are left in memory, while data is written to disk. (see below)
  • page compaction for reducing fragmentation over time.
  • multikey reads can be batched to reduce latency from multiple swaps to IO threads.
  • simple. uses a basic thread pool for IO's, suspends/resumes connections.
  • does not block if items are being SET faster than they can be flushed to flash. objects are instead evicted from tail of memory to make space. (reliable, predictable speed)
  • live control of what gets flushed to flash, ie: "objects >= 1024 bytes which are 3600s idle"


  • All data is tracked in memory. A restart of memcached effectively empties flash.
  • Objects leave a small part in memory + the original key. This means very small objects don't get any benefit from being flushed.
  • UDP is not be supported for objects flushed to disk.
  • Not a levelled (or any kind of sophisticated) database. It wants to be fast and forget things, rather than efficiently use 100% of disk space. Expect 80-90% best case.

Very high level detail

A single file is split into N logical pages of Y size (64M default). Write buffers (8M+) are used to store objects later flushed into pages.

IO threads are used to asynchronously flush write buffers and read items back.

Items are left in the hash table along with their keys, with small (24 byte as of this writing) headers detailing where data lives in flash.

The storage engine recycles pages by changing their version number. Item headers remember both the page ID and version number at the time the object was flushed. An attempted read with the wrong version will result in a miss.

The system is most useful if you have a breakdown of different item sizes: pools with a mix of small and large, or if you can break off a small pool of machines with flash devices to store much larger items. Freeing up more memory on your small item pools can dramatically improve hitrates, or simply reduce costs overall.


This is designed for maximum performance. It unifies into the existing hash table data structure in order to avoid burning CPU from shifting items into a secondary system. This also greatly reduces the IOPS required to maintain data.

IE: Rewriting an object does not touch flash. It simply "forgets" the position of the previous item on disk and informs the engine that particular page has fewer bytes stored. The same exists for deletes.

Recycling pages also does not require writing to them.

Compaction however does require rewriting pages. More details of that is in doc/storage.txt

How do I use it?

-o ext_path=/data/file,ext_page_count=100 this is enough to get you started. See memcached --help for other options. This wiki will update with more details on what the options do, but most users should be able to use a minimal subset of options. If you have a very high performance setup, feel free to reach out and we can walk you through a configuration.

The above creates an external cache of 6.4 gigabytes (100 * 64 meg page size default). Due to how the shim works, you wil not be able to actually store 6.4 gigs due to fragmentation over time. Background jobs will attempt to keep fragmentation in check.

How safe is this really?

I test pretty aggressively while developing features. The general stability of memcached should attest to a high quality. This feature is still in an experimental tages, but has had a good amount of baking time for the operations it does support. Keep in mind that many features are disabled for items sent to disk (incr/decr/append/prepend)

Technical Breakdown

This section is a detail on the architecture and tradeoffs of external storage.


Primarily: keeping "cache patterns" in mind, get the most out of a storage device by minimizing the accesses to it.

  • Larger values (typically 1k+) can use flash. Small values stay in RAM.
  • A single read to serve a single key.
  • Asynchronous batched writes to drive.
  • Main hash table is authoritative: miss/delete/overwrite must not use the drive.
  • Writing new items to cache must never block on the flash device.
    • Items are evicted from the LRU tail in RAM if extstore writer lags.
  • Forgetting / evicting data from storage must not use the drive.


Some assumptions about access patterns have to be made in order to utilize flash as a cache medium. Writing is expensive (destructive to the drive), and caches typically have much higher churn rate than databases. Flash also increases the latency of each key fetched, which can make caching small values on flash ineffective compared a range fetch against a high end database.

  • Longer TTL's for data that hits disk (and good reuse)
  • Latency overhead for small items is high compared to benefit of storing them.
  • Latency overhead for larger items is better than a miss (within reason)
  • A minority of data is hot (highly accessed), with a long tail of colder data.


A review of the main threads and data structures in memcached:

               THREADS            +            STRUCTURES
 +----------------------+         |
 |-----------|          |         |     +----------+
+------+  +--v-----+ +--v-----+   |     |hash table|
|listen|  |worker 1| |worker 2|   |     +----------+
+------+  +--------+ +--------+   |
                        +--------->     +---+
                                  |     |LRU|
+--------------+                  |     +---+
|lru maintainer+------------------>
+--------------+                  |     +--------------+
                                  |     |slab allocator|
+-----------+                     |     +--------------+
|lru crawler+--------------------->
+-----------+                     |
+---------------+                 |
|slab page mover+----------------->
+---------------+                 |


The listen thread accepts new connections, it then passes the socket to workers via writing a byte to a pipe. Each worker has its own "notifier pipe", which it receives events from via libevent polling. This mechanism is reused for extstore, allowing IO requests to pass to/from IO threads.

A client connection sticks to a worker thread for its lifetime. One worker thread per CPU is ideal.

The LRU maintainer thread moves items inbetween sub-LRU's (hot|warm|cold|etc). It also makes decisions on when to schedule page moves, or LRU crawls.

The LRU crawler reaps expired/invalid items, including items extstore invalidates.

Memory is divided into slab classes with fixed chunk sizes. Fixed size (1M) pages are assigned to each slab-class and divided into chunks as-needed. Over time, shifts in average item sizes or access patterns can "starve" a class. The page mover thread is able to safely re-assign pages between slab classes to avoid starvation.

Misc. threads: hash-table-expander, logger. These do not interact with extstore.


  • Hash table is chain bucketed. "next" is embedded in item memory.
  • LRU is doubly-linked, with next/prev embedded in item memory.
  • Each slab class has its own LRU.
  • Each LRU is split into HOT|WARM|COLD|TEMP

Now, with extstore:

               THREADS            +            STRUCTURES
 +----------------------+         |
 |-----------|          |         |     +----------+
+------+  +--v-----+ +--v-----+   |     |hash table|
|listen|  |worker 1| |worker 2|   |     +----------+
+------+  +--------+ +--------+   |
                  |     +---------+     +---+
                  |               |     |LRU|
                  |               |     +---+
                  |               |
+-------+       +-v--+   +----+   |     +--------------+
|storage+------->IO 1|   |IO 2|   |     |slab allocator|
+-------+       +----+   +-^--+   |     +--------------+
                           |      |
+----------+               |      |     +--------------------+
|compaction+---------------+      |     |extstore page memory|
+----------+                      |     +--------------------+
                                  |     +----------------------+
                                  +     |extstore write buffers|


The storage thread is similar to the LRU maintainer (they may merge later). It examines the LRU tails for each slab class. It requests a write buffer from extstore, CRC32's the item data, then hands the write buffer back to extstore.

The compaction thread targets pages ready for compaction. It reads the page data back from storage, walks the items within, and either throws them away or rewrites into a new page. See below for more detail.

Multiple IO threads exist which issue pread/pwrite/etc calls to the underlying storage on behalf of worker, storage, and compaction threads. Each IO thread has a simple stack queue with a mutex.

There is also an "extstore maintenance thread", which simply sorts page data for other threads. It may disappear and become an API call for the LRU maintainer.


Extstore has some of its own data structures. Mostly stats counters, and page data.

Pages on disk are (default 64M) subsections of a file or device. Each page has a small structure in memory. The structure in memory is used to avoid writing to flash when objects are deleted from a page. A page can be reclaimed once fully empty, so a page can be recycled without reading or writing back to it.

Extstore has write buffers which items are written into from the storage or compaction threads. When write buffers are full, a write will fail temporarily while the buffer is send to an IO thread. Once flushed, writes are accepted into the buffer again. This is used to rate limit writes to storage.

How an item flows to storage:

|LRU tail|
  | * Is the item large?               +------>
  |                                    |
  | * Is memory low?                   | Once written to buffer:
  |                                    |
  | * Is a write buffer available?     | * Allocate new small item
  |                                    |
  |   +------------+                   | * Copy key+metadata to new item
  +--->write buffer+-------------------+ * Also contains pointer to flash:
      +------------+                       [page, offset, version]

    * IF the write buffer was full:      * Replace original with small item

     +------------+     +---------+
     |write buffer+----->IO thread|
     +------------+     +---------+

    * All writes wait during flush
  • If all memory is exhausted while writes to storage are waiting, items will be evicted from the LRU tail in memory. IE: An item in slab class 10 waiting to be written to storage, will instead be evicted. Sets to cache must not block on storage.

How an item is read from storage:

|  +-------+
|  +-----------------+
+-->hash table lookup|

    If item is a flash header:

    * Create IO request for extstore

    * Continue parsing other keys in request

    Before response is written to the client,
    if it contains IO objects:

1                     4
   +-------------+     +-------------+
+--+worker thread<-----+libevent poll<----+
|  +-------------+     +-------------+    |
|                                         |
|  +---------+       +------------------+ |
+-->IO thread+------->worker notify pipe+-+
   +---------+       +------------------+
2                   3
  • With multigets, many IO objects can be sent to the IO thread at the same time. Typical flash devices are internally parallel, and can have similar latency for batched requests.

  • Workers fill iovec's with response data while parsing each key in the request. For every item header, blank iovec's are added where the data would be.

  • A temporary item is allocated from slab memory to hold the full item.

  • If an item has become invalid (page evicted, etc), the IO thread will record a miss back into the IO object before shipping back to the worker.

  • On a hit, the holes are filled in the iovec with the temporary item.

  • On a miss, the affected iovec entries are are NULL'ed out, wiping the response.

  • Temporary memory is freed at the end of the request.

Items read from storage have a chance of being recached into memory if they are read multiple times in less than a minute. This allows items which are only occasionally accessed to stay on storage. When they are recached into main memory, their flash entry is deleted.

The lifetime of a storage page:


       |     |
 0M    | P0  | 5
 64M   | P1  | 10
 128M  | P2  | 12
 192M  | P3  | 6
 256M  | P4  | 7
 320M  | P5  | 8
 384M  | P6  | 9
       |     |
       |     |
       |     |
       +     +

Storage pages are offsets into a file, with a default size of 64 megabytes per page. Future releases should allow multiple files (thus devices) be used for the same page pool. As of this writing, only buffered IO is used. O_DIRECT and async IO's are planned.

Each page has a version assigned to it as it is used. When a page is recycled or evicted, it is given a new version number.

Item headers in memory store: [PAGE ID, OFFSET IN PAGE, VERSION NUMBER]. The version number is used to validate an object is still valid when being fetched from storage.

Pages are organized into logical "buckets". In the extstore shim, buckets are arbitrary numbers. Memcached's higher levels give the buckets meaning




[P11]<----+LOW TTL+---->[P7,P8]

FREE: [P12,P13,...]

Buckets are used to coalesce data into related pages. By default all items are written into bucket 0 [DEFAULT].

  • If an item has been rewritten during compaction (see below), it is written into [COMPACTED] which are reserved for items having survived compaction.

  • If an item has a low remaining TTL at flush time (see ext_low_ttl option), it will be written into the [LOW TTL] bucket. This bucket is excluded from compaction. Pages are reclaimed periodically as the items expire.

[COMPACTED] pages tend to contain very long lived objects, so those pages have less fragmentation and need to be rewritten less often.

[LOW TTL] pages are never compacted, which avoids write amplification at the cost of some space over time.

In the future, [LOW TTL] may split into multiple buckets. Splitting IE: low, med, high, would allow very low pages to reclaim faster while still avoiding write amplification for higher TTL's.

How deletes, and overwrites work:

+--------+         +----+
|ITEM HDR+----+----+PAGE|
+--------+    |    +----+
[DELETE]      | [1048576 bytes stored]
              | [6000 objects stored]
* Remove from |
  hash table  | ---
              | [1048401 bytes stored]
              | [5999 objects stored]
[REPLACE]     |
              | ---
* Insert into |
  hash table  | No change

Pages track a few counters in memory only. The number of bytes stored, and the number of objects stored. A page has no way of retrieving what items are actually stored within it.

Deleting an object from a page reduces the object and byte count from the in-memory counters associated to that page. The same goes when page are being filled in the first place, the counters are incremented.

  • When bytes and objects in a page reach zero, it is immediately reaped and placed onto the free list.

  • Deletes or overwrites never cause new writes to happen to storage.

The compaction thread:

  • First, find a candidate to compact. IE: page is more than 50% empty. (tuned by ext_max_frag).

  • Since pages are written to aligned by write buffer size, the thread reads data back from the page in chunks the size of the write buffer (IE: 8M).

  • The buffer is walked linearly. Since items are fully written to storage, the read buffer can be parsed as item structures. Once an item is examined, the next one is found by advancing a pointer by item size.

  • When an item is examined:

    • The key hash is retrieved from storage pre-calculated, which speeds up readback.
    • An item lock is taken, and the hash table is checked for the original item.
    • If the item exists, and is still an item header which points into the same page at the same version, it's queued to be rewritten.
    • IF ext_drop_unread is enabled, a valid item is only rescued if it was recently accessed (specifically; if it's not in the COLD LRU).

drop_unread is important if a lot of data is being written through storage. This ensures objects which are getting requested, but not frequently enough to be recached into memory, aren't lost.

Memory thresholds:

The slab page balancer has an algorithm specific for extstore configurations.

For reference, a python and internal C version of the algorithm exist.

When tuning changes to the algoritm, it's easy to disable the internal algorithm and use external scripts.

Since the extstore system is designed to send items to storage before they're removed from memory, it needs to manage a buffer of free space. 1% or 0.5%.

  • In each slab class, 0.5% of chunks assigned to the class "should be free"
  • Overall, 0.5% of (1M) memory pages should be kept in the global page pool, to be assigned as needed.

These targets allow the storage thread some time to react to bursts of traffic without having to drop memory. If free memory drops below the threshold, the storage thread works to catch up.

  • Item headers use memory out of the global page pool, which ensures memory is available as items are pushed to storage.
  • If the global pool gets low, pages are reclaimed from the larger slab classes.

NOTE: The algorithm is currently does not prevent small items from taking all memory. If items are relatively small, or a very large amount of storage space is available compared to RAM, there will be no memory to hold other items. This will cause the flash to be hit much harder as very recent items are written out.

Caps may be introduced in the future.



  • Read amp.
  • Full readback during compaction.
  • Write amp.
  • No small items.
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