On Wed, Jan 15, 2014 at 8:41 PM, Jan Kara <jack@suse.cz> wrote:
> On Wed 15-01-14 10:12:38, Robert Haas wrote:
>> On Wed, Jan 15, 2014 at 4:35 AM, Jan Kara <jack@suse.cz> wrote:
>> > Filesystems could in theory provide facility like atomic write (at least up
>> > to a certain size say in MB range) but it's not so easy and when there are
>> > no strong usecases fs people are reluctant to make their code more complex
>> > unnecessarily. OTOH without widespread atomic write support I understand
>> > application developers have similar stance. So it's kind of chicken and egg
>> > problem. BTW, e.g. ext3/4 has quite a bit of the infrastructure in place
>> > due to its data=journal mode so if someone on the PostgreSQL side wanted to
>> > research on this, knitting some experimental ext4 patches should be doable.
>>
>> Atomic 8kB writes would improve performance for us quite a lot. Full
>> page writes to WAL are very expensive. I don't remember what
>> percentage of write-ahead log traffic that accounts for, but it's not
>> small.
> OK, and do you need atomic writes on per-IO basis or per-file is enough?
> It basically boils down to - is all or most of IO to a file going to be
> atomic or it's a smaller fraction?
The write-ahead log wouldn't need it, but data files writes would. So
we'd need it a lot, but not for absolutely everything.
For any given file, we'd either care about writes being atomic, or we wouldn't.
> As Dave notes, unless there is HW support (which is coming with newest
> solid state drives), ext4/xfs will have to implement this by writing data
> to a filesystem journal and after transaction commit checkpointing them to
> a final location. Which is exactly what you do with your WAL logs so
> it's not clear it will be a performance win. But it is easy enough to code
> for ext4 that I'm willing to try...
Yeah, hardware support would be great.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
