[dm-devel] all cache blocks marked as dirty in writethrough mode, "Data loss may occur", constant write activity

Joe Thornber thornber at redhat.com
Mon Jun 30 09:14:03 UTC 2014 

On Sun, Jun 29, 2014 at 12:54:53AM +0200, Marc Lehmann wrote:
> [I am not subscribed to the list, CC: appreciated]
> 
> Hi!
> 
> I tried to look for contact info for dm-cache bug reports, and decided to
> write to this list. If this is the wrong way to report errors, pointers on
> the correct way are appreciated. Also, if this report is bogus, I apologise
> in advance :)

This is the right place, don't worry.

> Anyway, I tried out dm-cache on debian kernel 3.14-0.bpo.1-amd64
> (3.14.7-1~bpo70+1) and 3.14-1-amd64 (3.14.7-1), both had essentially the
> same behaviour. I previously tried on a 3.12 kernel, which showed none of
> these issues.

I think your symptoms are due to us temporarily making the granularity
of the discard bitset match that of the cache block size.  This was a
quick fix for a race condition.  There are some big changes coming in
the handling of discard bios and we'll change it back then.

Let's break down your problems:

- It uses more metadata than expected.

  Yes, the discard bitset size is proportional to the size of origin,
  unlike the dirty bitset which is proportional to the cache size.
  Reducing the granularity has increased the space it takes up
  considerably.

- It assumes the cache is dirty if something went wrong during the
  last activation.

  Yes, constantly updating the dirty bitset on disk would have a large
  impact on IO latency.  So we skip it until you do a clean shutdown
  (ie. deactivate the device).  If a clean shutdown didn't occur then
  we assume all blocks are dirty and resync.

- There is an 18 second pause when removing the cache dev.

  IO will occur to the metadata device when you teardown the cache.
  This is when the dirty bitset and discard bitset get written.
  Increasing the discard bitset size has made this worse.  Though I
  think 18 seconds is way too long and will look into it.

- There's a constant 4k background load to the metadata device.

  I'll look into this.  Sounds like the periodic commit is re writing
  the superblock even if there's no change to the mappings.

- Joe


> 
> Namely, after creating a writethrough dm-cache mapping, removing it,
> and setting it up again, the whole cache is marked as dirty and written
> back to the origin device, which obviously shouldn't happen when using
> writethrough. I noticed this because my box was very sluggish for a while
> after each reboot (due to the hige write load).
> 
> On further inspection, I get kernel error messages on "dmsetup remove" of the
> dm-cache device:
> 
>    [ 3137.734148] device-mapper: space map metadata: unable to allocate new metadata block
>    [ 3137.734152] device-mapper: cache: could not resize on-disk discard bitset
>    [ 3137.734153] device-mapper: cache: could not write discard bitset
>    [ 3137.734155] device-mapper: space map metadata: unable to allocate new metadata block
>    [ 3137.734155] device-mapper: cache metadata: begin_hints failed
>    [ 3137.734156] device-mapper: cache: could not write hints
>    [ 3137.734159] device-mapper: space map metadata: unable to allocate new metadata block
>    [ 3137.734160] device-mapper: cache: could not write cache metadata.  Data loss may occur.
> 
> I used the formula "4MB + 16 * nr_blocks" to create the metadata device,
> so it shouldn't be too small (the cache device is 10G, blocksize is 64kb,
> and the calculated metadata partition has about 6MB).
> 
> I still get the above messages after increasing the metadata partition to
> 40MB. Only after increasing it to 70MB did the error go away, which also
> stopped all cache blocks to be marked as dirty.
> 
> Even with the 70MB metadata partition, behaviour is strange: dmsetup
> remove takes 18 seconds, with one cpu having 100% sys time with no I/O,
> and while the partitions are mounted, there is a constant 4kb write
> activity to each cache partition, with no activity on the origin partition
> (which causes ~1GB/day unnecessary wear).
> 
> Obviously dm-cache should not ever mark blocks as dirty in writethrough
> mode, and obviously, the metadata requirements are much higher than
> documented. Also, I think dm-cache should not constantly write to the
> cache partition when the system is idle.
> 
> Details:
> 
> All devices are lvm volumes.
> 
> I tried with both a 9TB and 19TB volume, both showed the same behaviour:
> 
>    RO    RA   SSZ   BSZ   StartSec            Size   Device
>    rw   256   512  4096          0   9499955953664   /dev/dm-7
>    rw   256   512  4096          0  20450918793216   /dev/dm-5
> 
> The cache devices are both 10G:
> 
>    RO    RA   SSZ   BSZ   StartSec            Size   Device
>    rw   256   512  4096          0     10737418240   /dev/dm-11
>    rw   256   512  4096          0     10737418240   /dev/dm-12
> 
> I use a script which divides the cache device into a 128kb header
> "partition", a metadata partition and a cache block partition. The working
> configuration is (the first line of each block is the cache partition
> mapping by lvm, followed by header/metadata/block mappings, followed by
> the cache mapping):
> 
>    vg_cerebro-cache_bp: 0 20971520 linear 8:17 209715584
>    cache-bp-header: 0 256 linear 253:12 0
>    cache-bp-meta: 0 144384 linear 253:12 256
>    cache-bp-cache: 0 20826880 linear 253:12 144640
>    cache-bp: 0 18554601472 cache 253:22 253:23 253:7 128 1 writethrough mq 2 sequential_threshold 32
> 
>    vg_cerebro-cache_wd: 0 20971520 linear 8:17 188744064
>    cache-wd-header: 0 256 linear 253:11 0
>    cache-wd-meta: 0 144384 linear 253:11 256
>    cache-wd-cache: 0 20826880 linear 253:11 144640
>    cache-wd: 0 39943200768 cache 253:16 253:17 253:5 128 1 writethrough mq 2 sequential_threshold 32
> 
> The configuration where the kernel complains about a too small metadata
> partition is:
> 
>    vg_cerebro-cache_bp: 0 20971520 linear 8:17 209715584
>    cache-bp-header: 0 256 linear 253:12 0
>    cache-bp-meta: 0 78848 linear 253:12 256
>    cache-bp-cache: 0 20892416 linear 253:12 79104
>    cache-bp: 0 18554601472 cache 253:22 253:23 253:7 128 1 writethrough mq 2 sequential_threshold 32
> 
>    vg_cerebro-cache_wd: 0 20971520 linear 8:17 188744064
>    cache-wd-header: 0 256 linear 253:11 0
>    cache-wd-meta: 0 78848 linear 253:11 256
>    cache-wd-cache: 0 20892416 linear 253:11 79104
>    cache-wd: 0 39943200768 cache 253:16 253:17 253:5 128 1 writethrough mq 2 sequential_threshold 32
> 
> If more details are needed, drop me a note.
> 
> Greetings,
> Marc Lehmann
> 
> -- 
>                 The choice of a       Deliantra, the free code+content MORPG
>       -----==-     _GNU_              http://www.deliantra.net
>       ----==-- _       generation
>       ---==---(_)__  __ ____  __      Marc Lehmann
>       --==---/ / _ \/ // /\ \/ /      schmorp at schmorp.de
>       -=====/_/_//_/\_,_/ /_/\_\
> 
> --
> dm-devel mailing list
> dm-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel

More information about the dm-devel mailing list