Thread: WALL on controller without battery?
Setting spec for a postgresql server. The hard drive distribution is going to be 8 x 750GB Seagate on a 3ware 9650SE RAID 6 2 x 160GB Seagate on a 3ware 2 port The question is, would I be better off putting WAL on the second, OS, controller or in the 8 port controller? Specially since the 2 port will not have battery (3ware does not have 2 ports with battery). The two port controller is primary for the operating system, but I was wondering if there would be any benefit to putting WAL on that 2 port controller. The machine will have 8Gb of RAM and will be running Postgresql 8.2 on FreeBSD 6.2 Stable. During peak operation there will be about 5 to 20 updates per second with a handfull of reads.
On Wed, Jul 11, 2007 at 10:48:04AM -0400, Francisco Reyes wrote:
> The question is, would I be better off putting WAL on the second, OS,
> controller or in the 8 port controller? Specially since the 2 port will not
> have battery (3ware does not have 2 ports with battery).
Put the WAL where the battery is. Even if it's slower (and I don't
know whether it will be), I assume that having the right data more
slowly is better than maybe not having the data at all, quickly.
A
--
Andrew Sullivan | ajs@crankycanuck.ca
The plural of anecdote is not data.
--Roger Brinner
On Wednesday 11 July 2007 08:36, Andrew Sullivan <ajs@crankycanuck.ca> wrote: > Put the WAL where the battery is. Even if it's slower (and I don't > know whether it will be), I assume that having the right data more > slowly is better than maybe not having the data at all, quickly. > Presumably he'll have the 2-port configured for write-through operation. I would spring for a 4-port with a BBU, though, and then put the WAL on the drives with the OS. -- "Bad laws are the worst sort of tyranny." -- Edmund Burke (1729-1797)
On Wed, 11 Jul 2007, Alan Hodgson wrote: > Presumably he'll have the 2-port configured for write-through operation. This is the real key to his question. In order to get acceptable performance for the operating system, Francisco may very well need the OS disks to be configured in write-back mode. If that's the case, then he can't put the WAL there; it has to go onto the array with the BBU. > I would spring for a 4-port with a BBU, though, and then put the WAL on the > drives with the OS. This is certainly worth considering. When putting multiple RAID controllers into a system, I always try to keep them of a similar grade because it improves the possibility of data recovery in case of a controller failure. For example, if he had a 4-port with BBU and an 8-port with BBU, the 8-port could be split into two 4-disk RAID-6 volumes, and then in an emergency or for troubleshooting isolation you could always get any data you needed off any 4-disk set with either controller. The little 2-disk unit is providing no such redundancy. -- * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
Alan Hodgson writes: > I would spring for a 4-port with a BBU, though, and then put the WAL on the > drives with the OS. The machine is already over budget. :-( I will check the price difference but unlikely I will get approval.
Francisco Reyes wrote: > Alan Hodgson writes: > >> I would spring for a 4-port with a BBU, though, and then put the WAL >> on the drives with the OS. > > The machine is already over budget. :-( > I will check the price difference but unlikely I will get approval. Without a BBU you are guaranteed at some point to have catastrophic failure unless you turn off write cache, which would then destroy your performance. Joshua D. Drake > > ---------------------------(end of broadcast)--------------------------- > TIP 7: You can help support the PostgreSQL project by donating at > > http://www.postgresql.org/about/donate > -- === The PostgreSQL Company: Command Prompt, Inc. === Sales/Support: +1.503.667.4564 || 24x7/Emergency: +1.800.492.2240 Providing the most comprehensive PostgreSQL solutions since 1997 http://www.commandprompt.com/ Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate PostgreSQL Replication: http://www.commandprompt.com/products/
Joshua D. Drake writes: > Without a BBU you are guaranteed at some point to have catastrophic > failure unless you turn off write cache, which would then destroy your > performance. I am re-working the specs of the machine to try and get a 4port 3ware to have the battery backup.
On Wed, 11 Jul 2007, Francisco Reyes wrote: > I am re-working the specs of the machine to try and get a 4port 3ware to have > the battery backup. That's really not necessary, it just would be better (and obviously more expensive). The warnings you've been getting here have been to let you know that you absolutely can't put the WAL on the controller with the OS disks attached without making compromises you probably won't be happy with. > During peak operation there will be about 5 to 20 updates per second > with a handfull of reads. There really is no reason you need to be concerned about WAL from a performance perspective if this is your expected workload. If you're working with a tight budget, the original design you had was perfectly fine. Just use all the disks on the big controller as a large volume, put both the database and the WAL on there, and don't even bother trying to separate out the WAL. If you expected hundreds of updates per second, that's where you need to start thinking about a separate WAL disk, and even then with 8 disks to spread the load out and a good caching controller you might still be fine. -- * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
Greg Smith writes: >> During peak operation there will be about 5 to 20 updates per second >> with a handfull of reads. > > There really is no reason you need to be concerned about WAL from a > performance perspective if this is your expected workload. I was able to get the second controller with battery backup. This machine is the backup so if the primary fails it would get higher volumes. It is also easier to throw more work at a good machine than to find myself with an underperformer. > both the database and the WAL on there, and don't even bother trying to > separate out the WAL. Thanks for the feedback. I wish there was a place with hardware guide where people could get feedback like the one you gave me. In particular actual numbers like x to y number of transactions per second you don't need WAL no separate disk.. etc..