All IO uncached when caching a partition #43

cachemonger opened this Issue Dec 19, 2011 · 8 comments


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I am trying to use flashcache to cache a partition (/dev/sdb1) using an SSD (/dev/sdd). Creating the cache works just fine, and I mount the new /dev/mapper/flashcache device in place of the original partition (instead of "mount /dev/sdb1 /mnt/target" I use "mount /dev/mapper/flashcache /mnt/target").

I see IO going through the flashcache driver when I watch the stats in /proc/flashcache/../flashcache_stats, but everything is listed as "uncached". I've double checked that I have cache_all enabled, and it is not set to filter sequential IO.

Everything works fine if I use the underlying /dev/sdb device instead (no partitions; create ext4 filesystem on /dev/sdb). If I do this, I see that IO is being cached.

The only thing I can think of is that sdb1 starts on the 63rd sector of sdb, and this is not 4K-aligned. Is this an unsupported configuration, or am I doing something else wrong?



Hi Mohans,

I'm not sure what you're asking. Do you want me to move the partition?


Hi all,
I have the same problem, everything seems pretty much aligned. Here are my stats, would appreciate any help.

# dmsetup table
fc-md: 0 2147481328 flashcache conf:
        ssd dev (/dev/sda), disk dev (/dev/disk/by-uuid/a6e4f3d2-12d1-41ae-b2f4-2f6782dab2c8) cache mode(WRITE_BACK)
        capacity(112640M), associativity(512), data block size(2048K) metadata block size(4096b)
        skip sequential thresh(0K)
        total blocks(56320), cached blocks(0), cache percent(0)
        dirty blocks(0), dirty percent(0)
Size Hist: 1024:1 4096:210445 8192:2165 12288:518 16384:2102 

# cat /proc/flashcache/flashcache_version 
Flashcache Version : flashcache-1.0
git commit: 1.0-139-g46eb0a330bcd

# dmsetup status
fc-md: 0 2147481328 flashcache stats: 
        reads(34015), writes(169867)
        read hits(0), read hit percent(0)
        write hits(0) write hit percent(0)
        dirty write hits(0) dirty write hit percent(0)
        replacement(0), write replacement(0)
        write invalidates(0), read invalidates(0)
        pending enqueues(0), pending inval(0)
        metadata dirties(0), metadata cleans(0)
        metadata batch(0) metadata ssd writes(0)
        cleanings(0) fallow cleanings(0)
        no room(0) front merge(0) back merge(0)
        disk reads(34022), disk writes(169893) ssd reads(0) ssd writes(0)
        uncached reads(34022), uncached writes(169893), uncached IO requeue(0)
        uncached sequential reads(0), uncached sequential writes(0)
        pid_adds(0), pid_dels(0), pid_drops(0) pid_expiry(0)

I did almost everything according to, but it seems there is an error in "-b 4096" param as "b" is in sectors, and 4096 sectors is 2048K. I will try to reconfig, thanks for pointing that out.


It worked. So, when creating device with flashcache_create and -b 4096, it makes block size equal to 4096 sectors, or 2/16 MB depending on your device sector size. When using -b 4k (which seems by default), block size is 4096 bytes. I corrected the wiki command. Thanks!

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