zram incompatibility

[onlyjob]

I'm getting the following errors in syslog when I'm trying to use zram0 block device as dm-writeboost cache:

kernel: [18820.076152] device-mapper: writeboost: read_superblock_header() I/O error(-5), bits(18446744073709551615), dev(252:1), sector(0), rw(0)
kernel: [18820.076157] device-mapper: writeboost: read_superblock_header failed
kernel: [18820.076158] device-mapper: writeboost: audit_cache_device failed
kernel: [18820.076287] device-mapper: table: 253:0: writeboost: resume_cache failed
kernel: [18820.076289] device-mapper: ioctl: error adding target to table

/dev/zram0 device is initialised as follows:

modprobe -v zram
echo 1024M > /sys/block/zram0/disksize

Is it possible to use dm-writeboost with RAM disk?

[onlyjob]

dm-writeboost can use /dev/loop0 on tmpfs as caching device.
Here is how 256M RAM disk can be initialised:

/bin/dd if=/dev/zero of=/tmp/ramloop0 bs=1048576 count=256
/sbin/losetup -v /dev/loop0 /tmp/ramloop0

[akiradeveloper]

Nice testing.

dm-writeboost can use /dev/loop0 on tmpfs as caching device.

Yes. I have tested with tmpfs before to emulate super-fast SSD. With the benchmark we could know how efficient the code paths of writeboost is.

I'm getting the following errors in syslog when I'm trying to use zram0 block device as dm-writeboost cache:

So, I am bit surprised to here this. From the user's point view, tmpfs and zram isn't different is my understanding; they are both just a block device.

This log says that read to super block (sector 0) failed and initialization of writeboost caching failed consequently. I am wondering if you can simply read/write the zram device by dd.

kernel: [18820.076152] device-mapper: writeboost: read_superblock_header() I/O error(-5), bits(18446744073709551615), dev(252:1), sector(0), rw(0)
kernel: [18820.076157] device-mapper: writeboost: read_superblock_header failed
kernel: [18820.076158] device-mapper: writeboost: audit_cache_device failed
kernel: [18820.076287] device-mapper: table: 253:0: writeboost: resume_cache failed
kernel: [18820.076289] device-mapper: ioctl: error adding target to table

[onlyjob]

I was surprised so much that I've opened this bug... I have no problem making swap on /dev/zram0 which actually works pretty much as normal block device except when I'm trying to use it with dm-writeboost...

[akiradeveloper]

I found similar issue but with dm-cache.

http://www.redhat.com/archives/dm-devel/2013-December/msg00114.html

I suppose this problem is relevant to device-mapper itself. Some code in device-mapper isn't suitable for using zram as underlying device for example.

[onlyjob]

Thank you for explanation. I forgot to answer your question -- yes I can read/write the zram device by dd or ddrescue.

[akiradeveloper]

I will post question to the dm-devel.

[onlyjob]

dm-writeboost works perfectly with brd (/dev/ramN) RAM disks hence I suppose it doesn't matter if zram disks can not be used.