I don't have anything significant to add to this conversation, but wanted
to chime in that I also find the concept of a QOS-like capability very
appealing and that Jim's recent emails resonate with me.  You're not alone!
 I believe there are many use cases where a granular prioritization that
controls how ARC, L2ARC, ZIL and underlying vdevs are used to give priority
IO to a specific zvol, share, etc would be useful.  My experience is
stronger in the networking side and I envision a weighted class based
queuing methodology (or something along those lines).  I recognize that
ZFS's architecture preference for coalescing writes and reads into larger
sequential batches might conflict with a QOS-like capability... Perhaps the
ARC/L2ARC tuning might be a good starting point towards that end?

On a related note (maybe?) I would love to see pool-wide settings that
control how aggressively data is added/removed form ARC, L2ARC, etc.
 Something that would accelerate the warming of a cold pool of storage or
be more aggressive in adding/removing cached data on a volatile dataset
(e.g. where Virtual Machines are turned on/off frequently).  I have heard
that some of these defaults might be changed in some future release of
Illumos, but haven't seen any specifics saying that the idea is nearing
fruition in release XYZ.


On Wed, Dec 5, 2012 at 10:26 AM, Jim Klimov <jimkli...@cos.ru> wrote:

> On 2012-11-29 10:56, Jim Klimov wrote:
>> For example, I might want to have corporate webshop-related
>> databases and appservers to be the fastest storage citizens,
>> then some corporate CRM and email, then various lower priority
>> zones and VMs, and at the bottom of the list - backups.
> On a side note, I'm now revisiting old ZFS presentations collected
> over the years, and one suggested as "TBD" statements the ideas
> that metaslabs with varying speeds could be used for specific
> tasks, and not only to receive the allocations first so that a new
> pool would perform quickly. I.e. "TBD: Workload specific freespace
> selection policies".
> Say, I create a new storage box and lay out some bulk file, backup
> and database datasets. Even as they are receiving their first bytes,
> I have some idea about the kind of performance I'd expect from them -
> with QoS per dataset I might destine the databases to the fast LBAs
> (and smaller seeks between tracks I expect to use frequently), and
> the bulk data onto slower tracks right from the start, and the rest
> of unspecified data would grow around the middle of the allocation
> range.
> These types of data would then only "creep" onto the less fitting
> metaslabs (faster for bulk, slower for DB) if the target ones run
> out of free space. Then the next-best-fitting would be used...
> This one idea is somewhat reminiscent of hierarchical storage
> management, except that it is about static allocation at the
> write-time and takes place within the single disk (or set of
> similar disks), in order to warrant different performance for
> different tasks.
> ///Jim
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