RE: Solid State Drives
- From: "Vishal Gupta" <vishal@xxxxxxxxxxxxxxx>
- To: <tanel@xxxxxxxxxx>, <dofreeman@xxxxxxxxxxx>, "Oracle-L" <oracle-l@xxxxxxxxxxxxx>
- Date: Sun, 3 May 2009 09:36:19 +0100
Tanel,
I would agree. SAN cache does a pretty good job, even with RAID-5 as we
have it in our bank.
Since most of the data, most of time is getting written to memory (RAM)
in SAN. And it offloaded to disk in the background by SAN. So database
gets a success handshake as soon as data is written to SAN cache. And
with combination of server RAM (i.e db cache) and SAN cache and RAID-5,
reads are also lot faster. As Tanel, suggests idead should be to
optimize your SQLs so they do less IO. But even if they cant, a full
tablescan might get repeatedly served from SAN cache.
Only problem I see with SAN cache is, there is no resource scheduling.
Its all shared. So if you have too many system on same SAN cache, then
one rouge system can bring down the entire company's systems. I have
seen that happening. If there is too much written to SAN cache and
writes are coming fast and thick. Then SAN does not get time to offload
this dirty cache to disk,and this write pending goes above your set
threshold, it starts throttling writes from all the sources. And we had
a linux kernel where it eventually made all the SAN connected mount
point read-only. OUCH.... Major downtime on all systems. Now linux
kernels have been patches so that mount point does not become read-only
under such conditions.
But SAN administrators really needs the understanding of databases IO,
and also needs to be contain busy system to their own front end ports
(Fibre channel ports) and their own disk and controllers.
But still they don't have the ability provided by SAN to isolate cache
for a particular system. Or ability to throttle only selected systems/FC
ports.
Regards,
Vishal Gupta
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Tanel Poder
Sent: 01 May 2009 18:22
To: dofreeman@xxxxxxxxxxx; 'Oracle-L'
Subject: RE: Solid State Drives
Once they get cheap and big then there would be a business case for them
for regular shops.
But right now, if you want to reduce the time spent waiting for physical
reads in your database - one way is buy faster IO subsystem which SSD
may give, another way is to just buy more memory for your server and do
less physical reads. The same is with writes, consider whether its
cheaper to buy/deploy/maintain the SSDs or just to have more write cache
in your storage array (and again, if you buy more RAM into your server
for caching read data then you can allocate even more of the storage
cache for caching writes).
So the question should be what's the most cost-effective option for
achieving the result - reducing TIME spent doing physical IO. Given the
write-caching of large storage arrays already in use in todays
enterprises I don't think adding SSDs make sense from cost/performance
perspective. Of course when the technology gets cheaper then the
potential power savings and lesser drive dying rate will be another
factor to consider.
So my prediction is that, unless some other major new technology emerges
in coming few years, SSDs will replace disk spindles for online "active"
data just like (remote) disk spindles have replaced tape backups in some
enterprises (btw I'm not saying that I like this approach entirely -
tapes have the benefit of being physically disconnected from any
servers, in a guarded safe in a bank in another city or so).
In addition to backups, the disk spindles will still be used for
archived data as well (lots of storage which is rarely accessed), they
are faster than tape but cheaper per gigabyte than SSDs. Long term
backups are kept on tapes, but some companies will completely throw away
their tape systems to cut costs & complexity and keep all backups on
disk spindles.
After saying all that - currently I don't see much reason for buying
SSDs for database solutions which are already deployed on mid/high-end
storage arrays.
--
Regards,
Tanel Poder
http://blog.tanelpoder.com <http://blog.tanelpoder.com/>
________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Freeman, Donald
Sent: 01 May 2009 16:09
To: Oracle-L (oracle-l@xxxxxxxxxxxxx)
Subject: Solid State Drives
Has anybody given any thought to where we are going as SSD's get
cheaper and bigger? We've been going round and round at my shop with
discussions about RAID, other disk allocation issues, fights over
storage. I mean we seem to spend a lot of time on that issue. I saw
that IBM is testing a 4 TB SSD. I was wondering if you'd have to
mirror that, What kind of reliability we would be getting. No more
RAID discussions? I've heard there is a finite number of times you can
write to it. What's the upgrade path here?
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