- applying compressor to segments older than 1 completed checkpoint ago -
saves space.
By the way: WAL compression is already implemented that way. If there are any ".zip" segments in archive dir, they are free to delete. This can be a safe workaround for users who experience lack of free space - just delete compressed segments. We should mention it in documentation for 2.4 release.

Best Regards,
Ivan Rakov

On 13.02.2018 23:53, Dmitry Pavlov wrote:
I see, it seems subgoal 'gain predictable size' can be achieved with
following options:
  - https://issues.apache.org/jira/browse/IGNITE-6552 implementation (in
variant of '...WAL history size in time units and maximum size in GBytes',
- here we probably should change description or create 2nd issue),
  - no-archiver mode ( segments still can be deleted, but in same directory
it was written) - maximum perfomance on ext* fs.
  - applying compressor to segments older than 1 completed checkpoint ago -
saves space.

Is it necessary to store data we can safely remove?

Or may be Ignite should handle this by itself and delete unnecessary
segments on low space left on device, like Linux decreases page cache in
memory if there is no free RAM left.

вт, 13 февр. 2018 г. в 23:32, Ivan Rakov <ivan.glu...@gmail.com>:

As far as I understand, the idea is WAL archive with predictable size
("N checkpoints" is not predictable size), which can be safely removed
(e.g. if free disk space is urgently needed) without losing crash recovery.

No-archiver mode makes sense as well - it should be faster than current
mode (at least, on filesystems different from XFS). It will be useful
for users who has lots of disk space and want to gain maximum throughput.

Best Regards,
Ivan Rakov

On 13.02.2018 23:14, Dmitry Pavlov wrote:
Hi, I didn't get the point why it may be required to separate WAL work,
WAL
uncheckpointed archive (some work outside segment rotation) and
checkpointed archive (which is better to be compressed using Ignite new
feature - WAL compressor).

Please consider new no-archiver mode implemented recently.

If archive folder confuses end user, grid admin may set up this mode (all
segments is placed in 1 directory) instead of introducing folders.


вт, 13 февр. 2018 г. в 22:11, Ivan Rakov <ivan.glu...@gmail.com>:

I think, I got the point now.
There's no need to copy files from "temp" to "archive" dir - we can just
move them, which is a constant-time operation.
Makes sense.

Change is quite complex (we need to synchronize all movings thoroughly
to avoid ruining existing WAL read iterators), but feasible.

Best Regards,
Ivan Rakov


On 13.02.2018 22:06, Ivan Rakov wrote:
Yakov,

This will work. However, I expect performance degradation with this
change. Disk storage has a limited number of I/O operations per second
on hardware level. List of already existing disk I/O activities
(writing to WAL work dir, copying from WAL work dir to WAL archive
dir, writing partition files during checkpoint) will be updated with a
new one - copying from WAL work dir to temp dir.

Best Regards,
Ivan Rakov

On 13.02.2018 21:35, Yakov Zhdanov wrote:
Ivan,

I do not want to create new files. As far as I know, now we copy
segments
to archive dir before they get checkpointed. What I suggest is to
copy them
to a temp dir under wal directory and then move to archive. In my
understanding at the time we copy the files to a temp folder all
changes to
them are already fsynced.

Correct?

Yakov Zhdanov,
www.gridgain.com

2018-02-13 21:29 GMT+03:00 Ivan Rakov <ivan.glu...@gmail.com>:

Yakov,

I see the only one problem with your suggestion - number of
"uncheckpointed" segments is potentially unlimited.
Right now we have limited number (10) of file segments with immutable
names in WAL "work" directory. We have to keep this approach due to
known
bug in XFS - fsync time is nearly twice bigger for recently created
files.

Best Regards,
Ivan Rakov


On 13.02.2018 21:22, Yakov Zhdanov wrote:

I meant we still will be copying segment once and then will be
moving it
to
archive which should not affect file system much.

Thoughts?

--Yakov

2018-02-13 21:19 GMT+03:00 Yakov Zhdanov <yzhda...@apache.org>:

Alex,
I remember we had some confusing behavior for WAL archive when
archived
segments were required for successful recovery.

Is issue still present?

If yes, what if we copy "uncheckpointed" segments to a directory
under
wal
directory and then move the segments to archive after checkpoint?
Will
this
work?

Thanks!

--Yakov




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