On 4/2/09 10:58 AM, "Merlin Moncure" <mmoncure@xxxxxxxxx> wrote: > On Wed, Mar 25, 2009 at 12:16 PM, Scott Carey <scott@xxxxxxxxxxxxxxxxx> wrote: >> On 3/25/09 1:07 AM, "Greg Smith" <gsmith@xxxxxxxxxxxxx> wrote: >>> On Wed, 25 Mar 2009, Mark Kirkwood wrote: >>>> I'm thinking that the raid chunksize may well be the issue. >>> >>> Why? I'm not saying you're wrong, I just don't see why that parameter >>> jumped out as a likely cause here. >>> >> >> If postgres is random reading or writing at 8k block size, and the raid >> array is set with 4k block size, then every 8k random i/o will create TWO >> disk seeks since it gets split to two disks. Effectively, iops will be cut >> in half. > > I disagree. The 4k raid chunks are likely to be grouped together on > disk and read sequentially. This will only give two seeks in special > cases. By definition, adjacent raid blocks in a stripe are on different disks. > Now, if the PostgreSQL block size is _smaller_ than the raid > chunk size, random writes can get expensive (especially for raid 5) > because the raid chunk has to be fully read in and written back out. > But this is mainly a theoretical problem I think. This is false and a RAID-5 myth. New parity can be constructed from the old parity + the change in data. Only 2 blocks have to be accessed, not the whole stripe. Plus, this was about RAID 10 or 0 where parity does not apply. > > I'm going to go out on a limb and say that for block sizes that are > within one or two 'powers of two' of each other, it doesn't matter a > whole lot. SSDs might be different, because of the 'erase' block > which might be 128k, but I bet this is dealt with in such a fashion > that you wouldn't really notice it when dealing with different block > sizes in pg. Well, raid block size can be significantly larger than postgres or file system block size and the performance of random reads / writes won't get worse with larger block sizes. This holds only for RAID 0 (or 10), parity is the ONLY thing that makes larger block sizes bad since there is a read-modify-write type operation on something the size of one block. Raid block sizes smaller than the postgres block is always bad and multiplies random i/o. Read a 8k postgres block in a 8MB md raid 0 block, and you read 8k from one disk. Read a 8k postgres block on a md raid 0 with 4k blocks, and you read 4k from two disks. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
