--
Dave Hall
Binghamton University
kdhall@xxxxxxxxxxxxxx
607-760-2328 (Cell)
607-777-4641 (Office)
On 11/25/16 10:20 AM, Dave Hall wrote:
On 11/25/16 6:18 AM, Carlos Maiolino wrote:
> > Regarding thing-provisioning, there are a couple things that
you should keep in
> > mind.
> >
> > - AGs segment the metadata across a whole disk, and increase
parallelism in the
> > filesystem, but, thin-provisioning will make such allocations
sequential,
> > despite where in the block device the filesystem tries to
write, this is the
> > nature of thin-provisioning devices so, I believe you should
be more careful
> > planning your DM-thin structure than the filesystem itself.
>
> So it sounds like I should used striping for my logical volume to
assure
> that data is distributed across the whole physical array?
I'm not sure if I understand you question here, what kind of
architecture you
have in your mind. All thin-provisioning allocation are sequential,
block
requested, next block available served (although with the recent
dm-thin versions
it will serve blocks in bundles, not on a block-by-block granularity
anymore,
but it is still a sequential alignment.
I am really not sure what you have in mind to 'force' the
distribution across
the whole physical array. The only thing I could think was to have 2
dm-thin
devices, on different pools, and use them to build a stripped LVM. I
don't know
if it is possible tbh, I never tried such configuration, but it's a
setup bound
to have problems IMHO.
Currently I have 4 LVM PVs that are mapped to explicit groups of
physical disks (RAID 5) in my array. I would either stripe or
concatenate them together and create a single large DM-Thin LV and
format it for XFS.
If the PVs are concatenated it sounds like DM-Thin would fill up the
first PV before moving to the next. It seems that DM-Thin on striped
PVs would assure that disk activity is spread across all of the PVs
and thus across all of the physical disks. Without DM-Thin, an XFS on
concatenated PVs would probably tend to organize an AGs into single
PVs which would spread disk activity across all of the physical disks,
just in a different way.
I'd like to add some clarification just to be sure...
The configuration strategy I've been using for my physical storage array
is to map specific disks into a small RAID group and define a single LUN
per RAID group. Thus, each LUN presented to the server is currently
mapped to a group of 5 disks in RAID 5.
If I understand correctly an LVM Logical Volume presents a single linear
storage space to the file system (XFS) regardless of the underlying
storage organization. XFS divides this space into a number of
Allocation Groups that it perceives to be contiguous sub-volumes within
the Logical Volume.
With a concatenated LV most AGs would be mapped to a single PV, but XFS
would still disperse disk activity across all AGs and thus across all
PVs. With a striped LV each AG would be striped across multiple PVs,
which would change the distribution of disk activity across the PVs but
still lead to all PVs being fairly active.
With DM-Thin, things would change. XFS would perceive that it's AGs
were fully allocated, but in reality new chunks of storage would be
allocated as needed. If DM-Thin uses a linear allocation algorithm on a
concatenated LV it would seem that certain kinds of disk activity would
tend to be concentrated in a single PV at a time. On the other hand,
DM-Thin in a striped LV would tend to spread things around more evenly
regardless of allocation patterns.
Please let me know if this perception is accurate.
Thanks.
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
