Re: Linux Software RAID5 - ballpark numbers

Rohan Beckles wrote:
Hello --

I realise this subject has been beaten to death, but I'd like to get a
handle on things. My basic system spec is in my sig. I'm running kernel
2.6.16.10.

I have recently built an additional array in the system, consisting of seven
250GB hard disks, configured for RAID 5 - total available storage is
1.46TB.

The array is split across two SIL3114A-based 4-channel PCI SATA controllers,
running in normal mode. One controller has 4 Hitachi 7K250 HDs, the other
has 2 Western Digital WD2500JD units and a single Seagate ST3250823AS.
[snip]
If I reconfigure the array as RAID0 (1.6TB capacity) I get an average
buffered write speed of around 12MB/s.

IME, Linux software RAID-0 should scale nearly linearly. I have a 2-drive RAID-0, using 15K RPM SCSI disks on a PCI-X U320 SCSI controller. Each disk is good for ~70 MB/sec, and the RAID-0 array does 140 MB/sec.

My first guess would be that you're seeing bus contention problems. The PCI SATA controllers are presumably both on the same PCI bus. The Firewire interface you mentioned for your external drive is almost certainly there too. Is there anything else on the PCI bus that might be eating up your bus bandwidth? (In the output of 'lspci', the digits before the first : are the bus number. Most systems only have a single PCI bus which includes built-in devices, too. Check yours.) Were any of those devices also in use at the time of your test?

(The RAID-5 performance could be affected by a number of additional factors. IMHO it's easier to start with sorting out the RAID-0 performance first.)
.

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