Re: I/O question

[mlelstv%serpens.de@localhost (Michael van Elst)]

wam%hiwaay.net@localhost ("William A. Mahaffey III") writes:

>> The RAID in question has 4 active drives, 1 parity drive & 1 spare, 
>> created from identical ~900 GiB partitions on each of 6 7200 RPM 1 TB 
>> SATA3 HDD's. Those drives purportedly ave platter I/O speeds of around 
>> 120 MiB/s (observed on other boxen). With 4 drive in parallel, that 
>> would be 480-ish MiB/s sustainable, under ideal conditions. I see 
>> about 11 MiB/s above. That implies somewhat non-ideal conditions, 
>> which might not be surprising :-/. I *thought* I setup the RAID for 
>> reasonably optimum performance during provisioning of the machine, as 
>> breath-takingly/tediously documented onlist. What sort of online 
>> diagnostics can I do (dumpfs, etc.) on the mounted filesystem to 
>> assess where I might reconfigure/tune the RAID for better performance. 

You need to check

- size and alignment of the RAID stripes
- size and alignment of the filesystem blocks

With 4 active drives (assuming 512Byte/Sector) you should:
- align the RAID partitions to a multiple of 128 sectors.
- use a 'sectPerSU' value of 32 (== 16kByte)
- create the FFS filesystem with a blocksize of 64kByte.

On some disks, using half the values (sectPerSU=16,blocksize=32kByte)
might be slightly better.

Directory and other metadata operations might still be slow.
You can avoid this by formatting the filesystem also with
a fragment size of 64kByte, but that will waste disk space.

WAPBL on such a disk will also have performance problems,
it might be necessary to set vfs.wapbl.flush_disk_cache=0
with a higher risk for data loss.

Write caching on the drives will also improve performance,
again with a higher risk for data loss.


-- 
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."