Hello,
The RAID5 configuration is: 8 SATA disks, 8 port Marvel SATA PCI-X
controller chip (SuperMicro board), Dual Xeon, 1 GB RAM, stripe size
64K, no spare disk.
Measurements are performed to the ram md device with:
disktest -PT -T30 -h1 -K8 -B65536 -ID /dev/md0
using the default stripe size (64K). 128K stripe size does not make a
real difference.
We have also increased the RAID 5 stripe cache by setting NR_STRIPES to
a larger value but without any perceptible effect.
If Linux uses "stripe write" why is it so much slower than HW Raid? Is
it disabled by default?
8 disks: 7 data disks + parity @ 64k stripe size = 448k data per stripe
The request size was smaller (tested up to 256K) than the size of a stripe.
We have seen errors for larger request sizes (e.g. 1 MB). Does Linux
require the request size to be larger than a stripe to take advantage
of "stripe write"?
Regards,
Mirko
Ming Zhang schrieb:
On Wed, 2005-08-24 at 10:24 +0200, Mirko Benz wrote:
Hello,
We have recently tested Linux 2.6.12 SW RAID versus HW Raid. For SW Raid
we used Linux 2.6.12 with 8 Seagate SATA NCQ disks no spare on a Dual
Xeon platform. For HW Raid we used a Arc-1120 SATA Raid controller and a
Fibre Channel Raid System (Dual 2 Gb, Infortrend).
READ SW:877 ARC:693 IFT:366
(MB/s @64k BS using disktest with raw device)
Read SW Raid performance is better than HW Raid. The FC RAID is limited
by the interface.
WRITE SW:140 ARC:371 IFT:352
For SW RAID 5 we needed to adjust the scheduling policy. By default we
got only 60 MB/s. SW RAID 0 write performance @64k is 522 MB/s.
how u test and get these number?
what is u raid5 configuration? chunk size?
Based on the performance numbers it looks like Linux SW RAID reads every
data element of a stripe + parity in parallel, performs xor operations
and than writes the data back to disk in parallel.
The HW Raid controllers seem to be a bit smarter in this regard. When
they encounter a large write with enough data for a full stripe they
seem to spare the read and perform only the xor + write in parallel.
Hence no seek is required and in can be closer to RAID0 write performance.
this is stripe write and linux MD have this.
We have an application were large amounts of data need to be
sequentially written to disk (e.g. 100 MB at once). The storage system
has an USV so write caching can be utilized.
I would like to have an advice if write performance similar to HW Raid
controllers is possible with Linux or if there is something else that we
could apply.
Thanks in advance,
Mirko
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