Re: Page Checksums

On Jan 24, 2012, at 9:15 AM, Simon Riggs wrote:
> On Tue, Jan 24, 2012 at 2:49 PM, Robert Treat <rob(at)xzilla(dot)net> wrote:
>>> And yes, I would for sure turn such functionality on if it were present.
>>>
>>
>> That's nice to say, but most people aren't willing to take a 50%
>> performance hit. Not saying what we end up with will be that bad, but
>> I've seen people get upset about performance hits much lower than
>> that.
> When we talk about a 50% hit, are we discussing (1) checksums that are
> checked on each I/O, or (2) checksums that are checked each time we
> re-pin a shared buffer? The 50% hit was my estimate of (2) and has
> not yet been measured, so shouldn't be used unqualified when
> discussing checksums. Same thing is also true "I would use it"
> comments, since we're not sure whether you're voting for (1) or (2).
>
> As to whether people will actually use (1), I have no clue. But I do
> know is that many people request that feature, including people that
> run heavy duty Postgres production systems and who also know about
> filesystems. Do people need (2)? It's easy enough to add as an option,
> once we have (1) and there is real interest.

Some people will be able to take a 50% hit and will happily turn on checksumming every time a page is pinned. But I suspect a lot of folks can't afford that kind of hit, but would really like to have their filesystem cache protected (we're certainly in the later camp).

As for checksumming filesystems, I didn't see any answers about whether the filesystem *cache* was also protected by the filesystem checksum. Even if it is, the choice of checksumming filesystems is certainly limited... ZFS is the only one that seems to have real traction, but that forces you off of Linux, which is a problem for many shops.
--
Jim C. Nasby, Database Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net