Re: fsync on ext4 does not work

* Havasvölgyi Ottó:

> 2011/12/19 Florian Weimer <fweimer(at)bfk(dot)de>
>
>> * Havasvölgyi Ottó:
>>
>> > Even though the TPS in pgbench about 700 with 1 client.
>> > I have tried other sync methods (fdatasync, open_sync), but all are
>> similar.
>> > Should I disable write cache on HDD to make it work?
>>
>> Did you mount your ext4 file system with the nobarrier option?
>>
>> By default, ext4 is supposed to cope properly with hard disk caches,
>> unless the drive is lying about completing writes (but in that case,
>> disabling write caching is probably not going to help much with
>> reliability, either).
>>
>
> It is mounted with defaults, no other option yet, so it should flush.
> These HDDs are 7200 rpm SATA with some low level software RAID1.
> I cannot understand why disabling HDD write cache does not help either.
> Could you explain please?

The drive appears to be fundamentally broken. Disabling the cache won't
change that.

But you mention software RAID1---perhaps your version of the RAID code
doesn't pass down the barriers to the disk?

> There is also an InnoDB transaction log on this partition, but its commit
> time is quite longer. On the same workload PgSql's commit is about 1 ms,
> but InnoDB's is about 4-7 ms. I think 4-7 is also too short to flush
> something to such disk, am I right?

Yes, it's still too low, unless multiple commits are grouped together.

--
Florian Weimer <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH http://www.bfk.de/
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