Re: [Xen-users] xen disk io performance

[Niels Dettenbach <nd@xxxxxxxxxxxx>]

Am Dienstag, 29. November 2011, 14:10:45 schrieb Jeff Sturm:
> Mechanical disks are slow.  The rules for disk performance haven't really
> changed due to virtualization: lots of RAM, lots of buffering.
ack
 
> In other words, avoid disk accesses like the plague.  Especially random I/O
> to physical drives.
ack.

> Solid-state storage helps, but you can often achieve the same effect with
> lots of RAM, on the cheap.
This is not correct in all cases as this still hardly depends from the model 
(wear leveling etc.) and - not at least - the applications disk usage profile 
(i.e. hardly parallel accesses etc.). This means: a high quality and 
application specific optimized SAS RAID could be significantly "faster" then 
many SSDs.

> random I/O is still the problem, which can only be mitigated with more 
spindles (we'd normally have at least 2 per physical host, but in our virtual 
cluster we've allocated one-fourth of that).
yes,
but just additionally: a "long" SAN path could slow down random i/o 
transaction rates too comparing to "directly" attached disks. Another "ugly" 
point on SAN could be parallel access from other users / SAN clients to the 
same physical disks or even swapping onto it from the same systems.

> Two, rotating media are 1970's technology, good for little more than 
archival, ripe for replacement.  I'm keeping an eye on price/capacity for 
solid state storage.

SSDs are still not a generally better / faster solution (even if you did not 
rely on the price) in each case - especially in database situations. Most 
database software developers / projects are still developing / optimizing 
their products for SSD storage usage as over decades for rotating disks before 
and the manufacturers of SSDs do their part of the development.

See i.e. developer discussions on MySQL etc. about this topic...


hth
best regards,


Niels.
-- 
---
Niels Dettenbach
Syndicat IT&Internet
http://www.syndicat.com/

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