On Sun, Aug 8, 2010 at 12:46 AM, Scott Carey <scott@richrelevance.com> wrote:
>
> On Aug 5, 2010, at 4:09 PM, Scott Marlowe wrote:
>
>> On Thu, Aug 5, 2010 at 4:27 PM, Pierre C <lists@peufeu.com> wrote:
>>>
>>>> 1) Should I switch to RAID 10 for performance? I see things like "RAID 5
>>>> is bad for a DB" and "RAID 5 is slow with <= 6 drives" but I see little on
>>>> RAID 6.
>>>
>>> As others said, RAID6 is RAID5 + a hot spare.
>>>
>>> Basically when you UPDATE a row, at some point postgres will write the page
>>> which contains that row.
>>>
>>> RAID10 : write the page to all mirrors.
>>> RAID5/6 : write the page to the relevant disk. Read the corresponding page
>>> from all disks (minus one), compute parity, write parity.
>>
>> Actually it's not quite that bad. You only have to read from two
>> disks, the data disk and the parity disk, then compute new parity and
>> write to both disks. Still 2 reads / 2 writes for every write.
>>
>>> As you can see one small write will need to hog all drives in the array.
>>> RAID5/6 performance for small random writes is really, really bad.
>>>
>>> Databases like RAID10 for reads too because when you need some random data
>>> you can get it from any of the mirrors, so you get increased parallelism on
>>> reads too.
>>
>> Also for sequential access RAID-10 can read both drives in a pair
>> interleaved so you get 50% of the data you need from each drive and
>> double the read rate there. This is even true for linux software md
>> RAID.
>
>
> My experience is that it is ONLY true for software RAID and ZFS. Most hardware raid controllers read both mirrors
andvalidate that the data is equal, and thus writing is about as fast as read. Tested with Adaptec, 3Ware, Dell PERC
4/5/6,and LSI MegaRaid hardware wise. In all cases it was clear that the hardware raid was not using data from the two
mirrorsto improve read performance for sequential or random I/O.
Interesting. I'm using an Areca, I'll have to run some tests and see
if a mirror is reading at > 100% read speed of a single drive or not.
