Re: sw raid0 read bottleneck

[Bill Davidsen <davidsen@tmr.com>]

Mark Hahn wrote:
>>>> 15 drives and 64K chunks gives 960K per stripe.
>>>> The raid0 code should set the read-ahead to twice that: 1920K
>>>> which I would have thought would be enough, but apparently not.
>>>
>>> choosing the RA size should depend in some way on speed, shouldn't it?
>>> after all, the goal is to have enough reads queued to avoid a "stall"
>>> while un-read-ahead-ed sectors pass under the head.  for a typical
>>> 60 MB/s, 7200 rpm disk, a track is .5 MB.  the heuristic above only 
>>> reads
>>> ahead .12 MB... 
>>
>> While I'm sure you realize it, some reader won't, so I'll point out 
>> that a cylinder is all the tracks which can be read without a seek, 
>> and cylinder size is track size times number of data heads. If you 
>> want max speed you can read that much off a drive at a time...
>
> hmm.  I certainly agree with your terminology, but afaikt, disks only 
> read
> from a head at a time.  the last time I saw a vendor quoting head-switch
> times, they were within a factor of two one-track seek times, which 
> explains
> why a same-family disk with more heads/surfaces shows only slight 
> speedup.
Agreed. The only real gain is avoiding intervening seeks. Makes a case 
for using cylinder size as chunk size? Maybe. Makes a case for being 
sure your readahead is large enough, though, for some sensible value of 
"enough."

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979