Re: defragmenting best practice?

["Tomasz Kłoczko" <kloczko.tomasz@gmail.com>]

On 14 September 2017 at 16:24, Kai Krakow <hurikhan77@gmail.com> wrote:
[..]
> Getting e.g. boot files into read order or at least nearby improves
> boot time a lot. Similar for loading applications.

By how much it is possible to improve boot time?
Just please some example which I can try to replay which ill be
showing that we have similar results.
I still have one one of my laptops with spindle on btrfs root fs ( and
no other FSess in use) so I could be able to confirm that my numbers
are enough close to your numbers.

> Shake tries to
> improve this by rewriting the files - and this works because file
> systems (given enough free space) already do a very good job at doing
> this. But constant system updates degrade this order over time.

OK. Please prepare some database, import some data which size will be
few times of not used RAM (best if this multiplication factor will be
at least 10). Then do some batch of selects measuring distribution
latencies of those queries.
This will give you some data about. not fragmented data.
Then on next stage try to apply some number of update queries and
after reboot the system or drop all caches. and repeat the same set of
selects.
After this all what you need to do is compare distribution of the latencies.

> It really doesn't matter if some big file is laid out in 1 allocation
> of 1 GB or in 250 allocations of 4MB: It really doesn't make a big
> difference.
>
> Recombining extents into bigger once, tho, can make a big difference in
> an aging btrfs, even on SSDs.

That it may be an issue with using extents.
Again: please show some results of some test unit which anyone will be
able to reply and confirm or not that this effect really exist.

If problem really exist and is related ot extents you should have real
scenario explanation why ZFS is not using extents.
btrfs is not to far from classic approach do FS because it srill uses
allocation structures.
This is not the case in context of ZFS because this technology has no
information about what is already allocates.
ZFS uses free lists so by negation whatever is not on free list is
already allocated.
I'm not trying to point that ZFS is better but only point that by
changing allocation strategy you may not be blasted by something like
some extents bottleneck (which sill needs to be proven)

There are at least few very good reason why it is even necessary to
change sometimes strategy from allocations structures to free lists.
First: ZFS free list management is very similar to known from Linux
memory SLAB allocator.
Did you heard that someone needs to do system memory defragnentation
because fragmented memory adds some additional latency to memory
access?
Other consequence is that with growing size of the files and number of
files or directories FS metadata are growing exponentially with size
and numbers of such objects. In case of free lists there is no such
growth and all structures are growing with linear correlation.
Caching in memory free list data takes much less than caching b-trees.
Last thing is effort on deallocating something in FS with allocation
structure and with free lists.
In classic approach number of such operations is growing with depth of b-trees.
In case free list all hat you need to do is compare ctime of the
allocated block with volume or snapshot ctime to make decision about
return or not block to free list.
No matter how many snapshots, volumes, files or directories allays it
will be *just one compare* of the block or vol/snapshot ctime.
With necessity to do just only one compare comes way better
predictable behavior of whole FS and simplicity of the code making
such decisions.
In other words ZFS internally uses well know SLAB allocator with
caching some data about best possible location to allocate some
different sizes allocation unit size multiplied by n^2 like you can
see on Linux in /proc/slabinfo in case of *kmalloc* SLABs.
This is why in case of ZFS number of volumes, snapshots has zero
impact on avg speed of interactions over VFS layer.

If you will be able present real impact of the fragmentation (again
*if*) this may trigger other actions.
So AFAIK no one been able to deliver real numbers or scenarios about
such impact.
And *if* such impact really exist one of the solutions may be just
mimic what ZFS is doing (maybe there are other solutions).

So please show us test unit exposing problem with measurement
methodology presenting pathology related to fragmentation.

> Bees is, btw, not about defragmentation: I have some OS containers
> running and I want to deduplicate data after updates.

Deduplication done in userspace has natural consequences in form of
security issues.
executable doing such things will need full access to everything and
needs to have exposed some API/ABI allowing fiddle with content of the
btrfs. Which adds second batch of security related risks.

Try to have look how deduplication is working in case of ZFS without
offline deduplication.

>> In other words if someone is thinking that such defragmentation daemon
>> is solving any problems he/she may be 100% right .. such person is
>> only *thinking* that this is truth.
>
> Bees is not about that.

I've been only trying to say that I would be really surprised if bees
will be taking care of such scenarios.

>> So first show that fragmentation is hurting latency of the
>> access to btrfs data and it will be possible to measurable such
>> impact. Before you will start measuring this you need to learn how o
>> sample for example VFS layer latency. Do you know how to do this to
>> deliver such proof?
>
> You didn't get the point. You only read "defragmentation" and your
> alarm lights lid up. You even think bees would be a defragmenter. It
> probably is more the opposite because it introduces more fragments in
> exchange for more reflinks.

So you are asking to start investing in the development time
implementing something without proving or demonstrating that problem
is real?
No matter how long someone will be thinking about this it will change nothing.

[..]
> Can we please not start a flame war just because you hate defrag tools?

Really I have no idea where I wrote that I hate defragmentation.
Using ZFS as working and real example I've only told you that
necessity to reduce fragmentation is NULL if you are following exact
path.
In your world you are trying to tell that you keys do not match to the
locker in doors.
I'm only trying to tell you that there are many doors without key hole
which can be opened and closed.

I can only repeat that to trigger some actions about defragmentation
first you need to *present* some case scenario exposing that the
problem is real. I may even believe you that you may be right but
engineering it is not something is possible to apply "believe" term.

Intuition always may be tricking you here that as long as impact is
non-zero someone should take care of this.
No. if this impact will be enough small this can be ignored as same as
we are ignoring some consequences of the quantum physics in our life
(probability that bucket of water standing on open fire may freeze
instead boil according to quantum physics is always non-zero and
despite this fact no one been able to observe something like this).
In other words you need to show some *real numbers* which will show
SCALE of the issue.

kloczek