[Gluster-users] Shard storage suggestions

[Krutika Dhananjay]

Hi,

The suggestion you gave was in fact considered at the time of writing shard
translator.
Here are some of the considerations for sticking with a single directory as
opposed to a two-tier classification of shards based on the initial chars
of the uuid string:
i) Even for a 4TB disk with the smallest possible shard size of 4MB, there
will only be a max of 1048576 entries
 under /.shard in the worst case - a number far less than the max number of
inodes that are supported by most backend file systems.

ii) Entry self-heal for a single directory even with the simplest case of 1
entry deleted/created while a replica is down required crawling the whole
sub-directory tree, figuring which entry is present/absent between src and
sink and then healing it to the sink. With granular entry self-heal [1], we
no longer have to live under this limitation.

iii) Resolving shards from the original file name as given by the
application to the corresponding shard within a single directory (/.shard
in the existing case) would mean, looking up the parent dir /.shard first
followed by lookup on the actual shard that is to be operated on. But
having a two-tier sub-directory structure means that we not only have to
resolve (or look-up) /.shard first, but also the directories '/.shard/d2',
'/.shard/d2/18', and '/.shard/d2/18/d218cd1c-4bd9-40d7-9810-86b3f7932509'
before finally looking up the shard, which is a lot of network operations.
Yes, these are all one-time operations and the results can be cached in the
inode table, but still on account of having to have dynamic gfids (as
opposed to just /.shard, which has a fixed gfid -
be318638-e8a0-4c6d-977d-7a937aa84806), it is trivial to resolve the name of
the shard to gfid, or the parent name to parent gfid _even_ in memory.


Are you unhappy with the performance? What's your typical VM image size,
shard block size and the capacity of individual bricks?

-Krutika

On Mon, Jul 18, 2016 at 2:43 PM, Gandalf Corvotempesta <
gandalf.corvotempesta at gmail.com> wrote:

> 2016-07-18 9:53 GMT+02:00 Oleksandr Natalenko <oleksandr at natalenko.name>:
> > I'd say, like this:
> >
> > /.shard/d2/18/D218CD1C-4BD9-40D7-9810-86B3F7932509.1
>
> Yes, something like this.
> I was on mobile when I wrote. Your suggestion is better than mine.
>
> Probably, using a directory for the whole shard is also better and
> keep the directory structure clear:
>
>
>  /.shard/d2/18/D218CD1C-4BD9-40D7-9810-86B3F7932509/D218CD1C-4BD9-40D7-9810-86B3F7932509.1
>
> The current shard directory structure doesn't scale at all.
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