On Apr 13, 2004, at 11:27 AM, scott.marlowe wrote:
> On Tue, 13 Apr 2004, Christopher Petrilli wrote:
>
>> 2. Retrieval time is limited not by disk bandwidth, but by I/O seek
>> performance. More spindles = more concurrent I/O in flight. Also, this
>> is where SCSI takes a massive lead with tag-command-queuing.
>>
>> In our case, we ended up using a three-tier directory structure, so
>> that we could manage the number of files per directory, and then
>> because load was relatively even across the top 20 "directories", we
>> split them onto 5 spindle-pairs (i.e. RAID-1). This is a place where
>> RAID-5 is your enemy. RAID-1, when implemented with read-balancing, is
>> a substantial performance increase.
>
> Please explain why RAID 5 is so bad here. I would think that on a not
> very heavily updated fs, RAID-5 would be the functional equivalent of a
> RAID 0 array with one fewer disks, wouldn't it? Or is RAID 0 also a
> bad
> idea (other than the unreliability of it) because it only puts the
> data on
> one spindle, unlike RAID-1 which puts it on many.
>
> In that case >2 drive RAID 1 setups might be a huge win. The linux
> kernel
> certainly supports them, and I think some RAID cards do too.
The issue comes down to read and write strategies. If your files are
bigger than the stripe size and begin to involve multiple drives, then
the rotational latency of each drive can come into play. This is often
hidden under caching during those wonderful comparison reviews, but
when you're talking about near random distributed access of more
information than could fit in the cache, then you have to face the
rotational issues of drives. Since the spindles are not locked
together, they drift apart in location, and you often end up with
worst-case latency in the drive subsystem. Mirroring doesn't face
this, especially when you can distribute the READS across all the
drives.
For example, if you ran triplex RAID-0, meaning 3 copies of the data,
which is often done in large environments so that you can take one copy
offline for a backup, while maintaining 2 copies online, then you can
basically handle 3 reads for the cost of 1, increasing the number of
read ops you can handle. This doesn't work with RAID-0, or RAID-5.
Chris
--
| Christopher Petrilli
| petrilli (at) amber.org
