On Wed 15-01-14 10:12:38, Robert Haas wrote: > On Wed, Jan 15, 2014 at 4:35 AM, Jan Kara <j...@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?
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... Honza -- Jan Kara <j...@suse.cz> SUSE Labs, CR -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers