Re: File system performance and pg_xlog (More info)

Well, as my tests continue, and I try to understand the nature of how file
system design affects Postgres, I did notice something disturbing.

On a single processor machine, Linux kernel 2.2x, and a good Adaptec SCSI
system and disks, the results were a clear win. When you put WAL on FAT32 on
its own disk, it ranges between 10% and 20% improvement.

My other machine, which is semi-production, I don't want to screw too much with
the OS, layout, etc. has a Paradise ATA-66 and two ATA-100 disks, which perform
quite well, usually. It is an SMP PIII 600, 512M RAM.

Using FAT32 was horrible, one tenth the performance of ext2. Perhaps this is
because FAT has one HUGE spinlock, where as ext2 has a better granularity? I
don't know, maybe I will get off my butt and examine the code later.

One thing is perfectly clear, file systems have a huge impact. While it may not
be an argument for writing a "pgfs," it is a clear indicator that optimal
performance is non-trivial and requires a bit of screwing around and
understanding what's best.

Personally, I would fear a "pgfs." Writing a kernel component would be a bad
idea. FAT has potential, but I don't think kernel developers put any serious
thought into it, so I don't think it is a mission critical component in most
cases. Just the behavior that I saw with FAT on SMP Linux, tells me to be
careful.

Postgres is at the mercy of the file systems, WAL make it even more so. My gut
tells me that this aspect of the project will refuse to be taken lightly.


-- 
I'm not offering myself as an example; every life evolves by its own laws.
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