Re: Best use of a small SSD with LVM

["John Stoffel" <john@stoffel.org>]

>>>>> "Brian" == Brian J Murrell <brian@interlinx.bc.ca> writes:

> I'm wondering what are the best practices for taking advantage of the
> speed of SSDs in combination with spinning rust drives in a volume
> group.

> I have 111GB of free SSD space in the same volume group as all of my
> system's filesystems (/, /usr, /var, /home, various other /var/cache/*
> filesystems, other data, etc.).

> Currently the 111GB PV on the SSD is completely unused.  But I wonder
> what the best use of it is.

> Should I just move entire VGs (i.e. something like /usr or /var or
> something else) on to it, or should I use it with LVM caching?  IOW is
> LVM caching effective?

> LVM caching seems like a lot of guesswork regarding how to split the
> SSD up most effectively so that you have the right size cache for
> the various volumes that you want to cache.  I.e. for every
> filesystem I have in my VG, I need to decide what percentage of the
> SSD I want to use for it and then how big I want the cache LV vs the
> cache metadata LV, etc.  Is there any way to monitor performance of
> the cache VGs to decide if they are being used effectively or if
> space should be moved from one cache LV to another?

I tried LVcache for quite a while when I had more HDDs with small
SSDs, and I didn't really find it make any visible different to me.  I
had my home directory and some scratch areas using this, but never
really got any wall time improvements.

Some of my tests were just reading emails and storing them on the
disk, starting/stopping applications.  Doing kernel compiles (git
pull, make oldconfig, make bzImage, etc) and it just didn't seem to do
much.  

> Can cache LVs be resized to optimize the size of them?  Perhaps I
> created a cache LV too big and it's not being used efficiently and
> then want to reduce it's size and use the freed up space to make a
> different cache LV bigger.

> Assuming one wants a writethrough cache because one does not have
> battery/UPS backed systems (such that they are vulnerable to power
> outages) (writethrough is the correct choice for that situation,
> correct?) is there any point to caching write-mostly filesystems such
> as /var/[log/] with a writethrough cache?

> Any other insights anyone wants to add?

I wouldn't bother honestly, I just never found it to be providing
meaningful performance.  But that's just me.