Re: SSD as Cache?

[David Christensen <dpchrist@holgerdanske.com>]

On 11/08/2013 12:12 PM, basti wrote:
> Sorry,
> /sbin/udevadm info --query=property --name=sdb |grep ID_MODEL
> ID_MODEL=M4-CT128M4SSD2
> MTBF 1,2 Mio h = 500000 days = 136,98 years
> Drive Endurance 72TB=40GB per day for 5 years
> I will try to measure the "traffic" next week.
> There are Hosted Servers, they are changed every 2 years to "new hardware".
> Here are Dedicated Root Servers.
> The Webapplication Files (PHP and so on are on RAID), also the Postgres
> Data-Dir.
> The Root Partition is on SSD, also /boot and swap.
> The Images are on SSD, "mounted via bind".
> The System has 32GB RAM and thats mainly used for Postgres cache and
> MongoDB.
> There is a Backend Postgres Database Server that cluster to this
> Frontend Server.
> So that this Frontend / Application Server can work "standalone".
> The mongoDB "cache" the Postgres Querys and some json Objects for the
> Application. Serialized Arrays and so on.
> When this crash it doesn't matter, It only takes longer to display the
> Webpages.
> I think 20-30 GB will be free on SSD when it's used as Cache, even more.
> (Thats my personal limit, that should be free on system disk.)

Please don't top-post.


Please trim included text.


That's a non-trivial configuration.  I infer two physical machines and 
at least four services from the above.  Your "Images" statement makes me 
wonder if you are also running hypervisor(s).  Are you?


Looking back to your original post:

> Now I plan to improve the performance of my Webapplication.
> The Cache is about 10.0 GB in 2000000 files.
> Can this cache moved to SSD?

Which cache are we talking about?


Where is the cache currently?


On 11/08/2013 03:46 PM, Karl E. Jorgensen wrote:
> Standard practice for tuning is to make measurements first.
> If your measurements show that disk IO is your bottleneck, then yes:
> SSD may help. But if your bottleneck is elsewhere, then changing
> things in the disk subsystem is unlikely to have any effect.

+1


Do you have any measurements?


David