Re: comparison with rival projects

[luvar-2S8i3W4CjuIzjkFyG6Beag@public.gmane.org]

First of all, I would like to thanks for answers.

One more question and some reply in text.

After making ssd disc a cache for some partition, there is possibility to detach cache. What should I do to "erase" (trim) whole disk for another benchmarking? I think that "dd if=/dev/zero of=/dev/sdx" wouldnt be right command.

----- "Nauman Rafique" <nauman-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:

> On Wed, Dec 15, 2010 at 4:05 PM, Kent Overstreet
> <kent.overstreet-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > On 12/15/2010 03:26 PM, luvar-2S8i3W4CjuIzjkFyG6Beag@public.gmane.org wrote:
> >>
> >> Hi,
> >> Iam new in problematic of caching persistent data. I want to use
> caching
> >> (on ssd disk) on my desktop with a lot of ram. I hope, it will
> provide me
> >> better latency for starting programs (developer/scientific
> desktop). My
> >> questions are:
> >>
> >> Is it possible to use single ssd disk for making two caches? One
> for
> >> /dev/md2 (root partition, raid10) and second for /dev/md3
> (raid5,home
> >> partition, etc) (I hope this is possible by partitioning that ssd
> disk and
> >> using partitions for caching)
> >
> > Yep. One cache device can be used with (currently) up to 256
> backing
> > devices.
> >
> >>
> >> Is it dangerous (in terms of data loss) to use writethrough caching
> to
> >> single ssd disk of raid5 block device? (I have 5 disks, using lvm
> on top of
> >> raid).
> >
> > Shouldn't be, recovery from unclean shutdown is quite well tested,
> both
> > writethrough and writeback should be perfectly safe.
> >
> >>
> >> For caching device is most critical number for random reads or
> operations
> >> per second. Is it right?
> >
> > For writethrough caching random reads, for writeback both reads and
> writes.
> >
> >>
> >> Trim is not need feature for cache ssd device. It will have same
> cache
> >> performance without it. Is it right?
> >
> > Really just depends on the ssd; some (cheaper, earlier) drives are
> known for
> > performance significantly degrading over time, with trim helps with
> (but
> > doesn't necessarily eliminate). Bcache uses trim if it's available.
> >
> >>
> >> What benefits/disadvantages have bcache [1] project over dm-cache
> [2] and
> >> flashcache [3]? My current knowledge is that writethrough is
> possible only
> >> in bcache, but dm-cache is implemented in more "standard" way
> (thdough
> >> devicemaper). Also bcache has some more information on web.
> >>
> >> [1] - http://bcache.evilpiepirate.org/
> >> [2] - http://users.cis.fiu.edu/~zhaom/dmcache/index.html
> >> [3] - https://github.com/facebook/flashcache
> >
> > Flashcache is based off of dm-cache.
> >
> > Flashcache has been used in production awhile, bcache is still a
> little
> > rough around the edges - but bcache has better performance, more
> features,
> > and it always orders writes correctly so as to be crash safe
> (flashcache has
> > a "torn write" problem).
> 
> Kent, now that we are on the topic, I wonder if there has been some
> benchmarking comparing bcache to flashcache performance?
> 
> Also I wonder if you plan to target for getting bcache included in
> mainline kernel. I wonder if such attempts were made for flashcache
> but that's probably off-topic.

I hope, next two links will help in making comparison.
http://www.mysqlperformanceblog.com/2010/05/10/flashcache-first-experiments/
http://www.mysqlperformanceblog.com/2010/05/18/flashcache-more-benchmarks/
There is simple methodology of cache testing by using it with mysql database. I hope, someone will catch this discussion and try some comparable test on bcache.

For me will be next test sufficient:
Activate caching and make for example 20 reboots. Measure time from computer bios beep till starting gimp, which will be in gnome/kde/fluxbox autostart script. Of course before starting gimp, there will be need for starting some other programs like firefox, eclipse, mplayer with some film...

I would like to see these times of booting in graph. I promise that after installation of bcache I will do this test with bcache and post it here.

> 
> Thanks.
> --
> Nauman
> 
> >
> >>
> >> PS: Is there any tutorial for gentoo users?
> >
> > Nah, I'm an ubuntu/debian user. For caching / the important thing is
> to hook
> > into your initramfs and get everything loaded before you mount your
> root
> > filesystem.
> >
> >>
> >> Thanks for any explanations / answers,
> >> --
> >> LuVar
> >> --
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> >

PS: I am now decided to use bcache (after buying some ssd).
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