[Btrfs-devel] Re: Checksum verify failed on btrfs-unstable.

[Chris Mason]

On Monday 24 March 2008, Evgeniy Polyakov wrote:
> Hi.
>
> During stress iozone test I got folowing in the dmesg:
> btrfs: sdb1 checksum verify failed on 339968 wanted 0 found 4DF48705
> from_this_trans 0

These are harmless for now, there is a small window where a buffer can change 
while it is being written (making the csum invalid).  It is slated to be 
fixed after the multi device code goes in.

>
> Performance degraded to hell for this test (8gb file, automatic iozone
> mode, probably during random read test).

I'd have to see all the test details to explain performance problems.  I would 
expect that once you do a series of random writes to the file, sequential 
reads are going to be painfully slow.  For that kind of database workload, 
you'll want to mount with -o nodatacow.  Since iozone lays out the file once 
in the beginning, performance will definitely suffer after it does a random 
write run.

>
> Filesystem was made with -l 4k -n 4k options and mounted with
> noatime,nodatasum,ssd options.
>
> Single 266 gb scsi disk.
> Any additional details?

The ssd mount option assumes that reads are seek free.  It will be optimal for 
postmark, which only does random reads and never does sequential reads, but 
it will be slower for most other benchmarks.

-chris