[CentOS] 4 X 500 gb drives - best software raid config for a backup server?
Kay Diederichs
kay.diederichs at uni-konstanz.de
Sat Feb 21 09:59:42 UTC 2009
Chan Chung Hang Christopher schrieb:
>> "md1 will read from both disk" is not true in general.
>> RAID1 md reads from one disk only; it uses the other one in case the
>> first one fails. No performance gain from multiple copies.
>>
> I beg to differ. I have disks in a raid1 md array and iostat -x 1 will
> show reads coming off both disks. Unless you do not have the multipath
look more carefully - with the current 2.6.18-9.1.22 kernel the bulk of
the data are read from one of the disks
> module loaded, md will read off both disks. Now whether md will read
> equally off both disks, that certainly will not be true in general.
>> You can easily see this for yourself by setting up a RAID1 from e.g.
>> sda1 and sdb1 - /proc/mdstat is:
>>
>> Personalities : [raid1]
>> md1 : active raid1 sdb1[1] sda1[0]
>> 104320 blocks [2/2] [UU]
>>
>> and then comparing the output of hdparm -tT :
>>
> ROTFL.
>
> How about using the proper tool (iostat) and generating some disk load
> instead?
hdparm -tT tests one type of disk access, other tools test other
aspects. I gave the hdparm numbers because everyone can reproduce them.
For RAID0 with two disks you do see - using e.g. hdparm - the doubling
of performance from two disks.
If you take the time to read (or do) RAID benchmarks you'll discover
that Linux software RAID1 is about as fast as a single disk (and RAID0
with two disks is about twice the speed). It's as simple as that.
>
>> To get performance gain in RAID1 mode you need hardware RAID1.
>>
>
> Bollocks. The only area in which hardware raid has a significant
> performance advantage over software raid is raid5/6 given sufficient
> cache memory and processing power.
We were talking about RAID1; RAID5/6 is a different area. Linux software
RAID1 is a safeguard against disk failure; it's not designed for speed
increase. There is a number of things that could be improved in Linux
software RAID; read performance of RAID1 is one of them - this _is_ why
some hardware RAID1 adapters indeed are faster than software.
Read http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing - since
the 2.6.25 kernel a simple alternating read is implemented, but that
does not take the access pattern into account.
So Linux software RAID1 is just mirroring - and it's good at that.
HTH,
Kay
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