[Gluster-users] RAM/Disk ratio question

[Paul Robert Marino]

Unfortunately that kind of tuning doesn't have any simple answers, and
any one who says there is should not be listened to.

It really depends on your workload and a lot of other factors such as
your hardware. for example a 20 plater RAID 1+0 on spinning disks with
a wide stripe needs very little cache for streaming large (MultiGB)
files due to the large IOPS they can do, but would need a large cache
for lots of files smaller than the stripe due to the fact that each
file access is a minimum of 1 IOP which means a full read of the
stripe. The reverse may be true if the files are only 4k or less on
average, in which case a standalone SATA SSD would be way faster and
need very little cache,but on large (MultiGB) files it would need a
huge amount of cache due to the 4k per IOP size limitation in SSD's.
Furthermore those scenarios assume your filesystem is correctly
aligned, unfortunately they usually aren't. The reasons for this are
complicated but in short the drivers (and in many cases the chipsets)
for many RAID and SATA controllers do not provide the information the
OS (/sys, LVM, and the filesystem) requires to align the filesystem
automatically when its created.
Now most DBA's will tell you they need an insane number of IOPs, what
they are really telling you is how many operations the database is
doing, not how many IOP's its doing. In reality databases do
surprisingly few IOPs and tend more to do large (MultiGB) sequential
reads into the ram used by the database processes, then do all their
operations there.

Also an other key factor is the IO scheduler (elevator="....." in the
kernel boot options) you are using in the kernel. CFQ which is the
default is great for desktops and servers running the 10 or more
different services on inexpensive hardware. on most dedicated servers
deadline or possibly if you have a good raid controller noop is much
better. using the proper IO scheduler can have a dramatic impact on
how much ram you use for cache, especially for writes.

As I said there is no easy answer to this but if you can give us an
idea of the typical workload then we may be able to give some advice.



On Fri, May 27, 2016 at 8:27 AM, kostas makedos
<kostas.makedos at gmail.com> wrote:
> Hello,
> Can someone give me an estimate ratio between RAM consumption in
> a node in respect to the GB stored in its bricks?
> Is there a rule of thumb or a guideline document?
>
>
> Thank you,
>
> Best Regards
> Kostas Makedos
>
> kostas.makedos at gmail.com
>
>
>
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