Aleksandar Milivojevic <alex@xxxxxxxxxxxxxxx> wrote:
> Correct me if I'm wrong, but you got 233MB/s for reads (the
> block read test).
Oh, good catch! I didn't even see that when responding (I
assumed he could interpret the bonnie benchmark). And if I
see that correctly, that was with a 12GiB file (on a system
that had 6GiB RAM).
> Assuming your disks can do 50MB/s sustained transfer rate
> each, you are preatty darn close to the theoretical maximum
> of (6 - 1) * 50MB/s = 250MB/s for 6 disk RAID5.
On reads, yes. 3Ware is clearly leveraging the ASIC's
non-blocking I/O for reads from RAID-5, which basically act
like RAID-0.
> RAID5 as such is bad choice for file systems that will have
> more than about 30% of writes (out of total I/O).
He still should be seeing at least 100MBps for RAID-5 writes
on a 3Ware Escalade 9500S with 6-discs (180MBps is about the
maximum for RAID-5 writes on the 9500S' ASIC with DRAM). The
ASIC is fairly good at sequential writes to RAID-5, and there
is enough DRAM to buffer all but the heaviest of random I/O.
Still, the new 9550SX series has a PowerPC. AMCC's influence
is clearly being pressed on their 3Ware acquisition, as they
are _the_ company for the IBM embedded PowerPC 400 line now.
The 9550SX is supposed to be cable of 380MBps for RAID-5
writes -- double the 9500S best benchmarks.
> If most of the I/O will be writes, and you care about
> performance, you should use RAID-10.
Yep, mega-dittos on that point.
> Remember, writes to Dumb, not-optimized RAID5
> implementation is slower than writing to a single
> disk. This is generic RAID wisdom, nothing to do with
> any particular implementation. In the worst case scenario,
> the write operation on 6-disk RAID5 volume involves
> reading a data block from 5 drives, calculating XOR,
> and writing back one block of data and one block of
> checksum. Whichever way you do it, it ain't gonna be
> fast.
Still, he shouldn't be seeing less than 100MBps writes on the
3Ware Escalade 9500S series with its on-board ASIC and DRAM
buffer.
At least the reads are very accurate for his configuration.
I'm curious how he is striping though? It might have been
better to do a 12-disc RAID-5 and get close to 400MBps reads.
Or if performance was more important than efficiency, making
one 6-disc volume RAID-10 would give close to 300MBps reads,
150MBps writes -- maybe higher.
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