Hi David Sommerseth!

 On 2015.04.14 at 12:55:14 +0200, David Sommerseth wrote next:

> Considering that the RAID technology is old which was designed for
> rotating media, maybe that isn't the the best solution for SSD?
> I'm wondering ... maybe another (possibly better?) approach would be
> dm-cache?  This has been added as a tech-preview in RHEL7.

Heh, RAID is VERY old and doesn't play that good with modern rotating
media as well, which is why RAID2 and RAID3 are obsolete for a long
time, and RAID5 is obsolete / too dangerous for a few years by now; not
to mention write hole problems of RAID5/6, no solution for data
inconsistency problems between drives for RAID1/10 and other issues.

The proper modern era RAID replacements would be software volume-based
solutions, preferable integrated into filesystem (zfs, btrfs) and
distributed filesystems like ceph. That aside, using RAID1 or 10 for
very small servers to increase reliability somewhat and RAID6 and
similar for big file storages and backup servers still does make some
sense..

> The purpose of dm-cache is to have a faster unit (such as an SSD) to
> cache contents from a slower device (such as a ordinary hard drive).  To
> have better data security, the ordinary hard drive(s) can be an md-raid
> device.
> 
> Maybe dm-cache can provide the best from SDD, HDD and RAID combined?

Not really. It was designed for completely different usage (think of it
as of ability to greatly increase file cache size, cheaper than adding
more RAM). It doesn't solve problems which are solved by RAID and
doesn't have its performance characteristics as well.

Using second drive for increased reliability to protect against device
failure: RAID1 can do this, dm-cache cannot (hard drives breaks - you
lose data even if it was somewhat cached on SSD).

Using multiple drives for creating storage of a big size - somewhat
slow, but with good reliability: RAID5/6/50/60 can do this, dm-cache
cannot.

Using multiple drives for getting guaranteed high bandwidth (required
for video recording from multiple stream sources, video editing and
such): RAID0 can do this, dm-cache cannot.

But yes, for course it makes sense to use dm-cache with RAID, e.g. on
database server where you can't afford to go all-SSD or on file storage
where you need extra performance for popular accessed files.

-- 

Vladimir