Re: Help: very slow software RAID 5.

[Bill Davidsen <davidsen@tmr.com>]

Dean S. Messing wrote:
> Justin Piszcz wrote:
> : One of the 5-10 tuning settings:
> : 
> : blockdev --getra /dev/md0
> : 
> : Try setting it to 4096,8192,16384,32768,65536
> : 
> : blockdev --setra 4096 /dev/md0
> : 
> :
>
> I discovered your January correspondence to the list about this.  Yes,
> the read-ahead length makes a dramtic difference---for sequential data
> reading.  However, in doing some further study on this parameter, I
> see that random access is going to suffer.  Since I had intended
> to build an LVM on this RAID 5 array and put a full linux system on
> it, I'm not sure that large read-ahead values are a good idea.
>
> : Also, with a 3-disk raid5 that is the worst performance you can get using 
> : only 3 disks
>
> I don't know why (for reads) I should suffer such bad performance.
> According to all I've read, the system is not even supposed to read
> the parity data on reads.  So why do I not get near RAID 0 speeds w/o
> having to increase  the Read-ahead value?
>
> : , while with a 10 disk raid5 it'd be closer to 90%.  Reads you 
> : should get good speeds, but for writes-- probably not.
>
> When you say "reads you should get good speeds" are you referring
> to the aforementioned 10 disk RAID 5 or my 3 disk one?
>
> : Then re-benchmark.
>
> Large Read-ahead nearly double the speed of 'hdparm -t'.  (So does
> simply using the "--direct" flag.  Can you explain this?)  Also, in
> your opinion, is it wise to use such large read-aheads for a RAID 5
> intended for the use to which I plan to put it?
>   

Do you want to tune it to work well now or work well in the final 
configuration? There is no magic tuning which is best for every use, if 
there was it would be locked in and you couldn't change it.
> Aside: I have found RAID quite frustrating.  With the original two
> disks I was getting 120-130 MB/s in RAID 0.  I would think that for
> the investment of a 3rd drive I ought to get the modicum of
> redundancey I expect and keep the speed (at least on reads) w/o
> sacrificing anything.  But it appears I actually lost something for my
> investment.  I'm back to the speed of single drives with the modicum
> of redundancey that RAID 5 gives.  Not a very good deal.
RAID-5 and RAID-1 performance are popular topic, reading the archives 
may shed more light on that. After you get to LVM you can do read ahead 
tuning on individual areas, which will allow you to do faster random 
access on one part and faster sequential on another. *But* when you run 
both types of access on the same physical device one or the other will 
suffer, and with careful tuning both can be slow.

When you get to the point where you know exactly what you are going to 
do and how you are going to do it (layout) you can ask a better question 
about tuning.

PS: adding another drive and going to RAID-10 with "far" configuration 
will give you speed and reliability, at the cost of capacity. Aren't 
shoices fun?

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