----- Original Message ----- > From: "Greg Smith" <g...@2ndquadrant.com> > To: pgsql-general@postgresql.org > Sent: Friday, 22 April, 2011 12:49:28 AM > Subject: Re: [GENERAL] SSDs with Postgresql? > On 04/20/2011 01:50 AM, Toby Corkindale wrote: > > Also, the number of erase cycles you can get, over the whole disk, > > is > > quite large on modern disks! > > > > So large that you'll probably go decades before you wear the disk > > out, > > even with continual writes. > > > > Don't buy into the SSD FUD myths.. > > There is no FUD being spread here. Particularly given the PostgreSQL > WAL write pattern, it's not impossible to wear out a SSD placed there > in > a small number of years. A system with a trivial but not completely > idle workload will generate one 16MB WAL segment every 5 minutes, > which > works out to 4.5GB/day of writes. That's the baseline--the reality is > much, much higher than that on most systems. The fact that every row > update can temporarily use more than 8K means that actual write > throughput on the WAL can be shockingly large. The smallest customer I > work with regularly has a 50GB database, yet they write 20GB of WAL > every day. You can imagine how much WAL is generated daily on systems > with terabyte databases.
The larger the database, the larger number of disks the DB will be spread over, thus keeping the amount of data written per-disk to a manageable amount. (Also, larger SSDs seem to have higher write-limits too) I see Intel is/was claiming their SLC SSDs had a *minimum* lifetime of 2PB in writes for their 64GB disks; for your customer with a 50GB db and 20GB/day of WAL, that would work out at a minimum lifetime of a million days, or about 273 years! The cheaper "consumer grade" MLC drives should still last minimum 5 years at 20GB/day according to their literature. (And what I found was fairly out of date) That doesn't seem too bad to me - I don't think I've worked anywhere that keeps their traditional spinning disks in service beyond 5 years either. > As for what this translates into in the real world, go read > http://archives.postgresql.org/message-id/BANLkTi=GsyBfq+ApWPR_qCA7AN+NqT=z...@mail.gmail.com > as one worked out sample. Anyone deploying PostgreSQL onto MLC can't > necessarily ignore this issue. Agreed, it shouldn't be ignored, but it seems like you have to have an early-generation SSD and combine that with very heavy write loads in order to risk wearing them out within a matter of years - and there are SMART attributes that should be able to indicate when the drive has used a lot of its life-span as an early-warning signal. You've linked to one anecdotal failure, but surely there are many more people using SSDs successfully, who just haven't posted about it because it's working fine..? -Toby -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
