[Borgbackup] Decreasing speed of borg check command?

[Thomas Waldmann]

> I would like to consult one observation. Performing a 600GB+ repo check,
> the HDD read speed started at ~110MB/s (USB3 HDD drive), but over time
> tends to decease up until ~60MB/s after 2 hours. Disk transfer data from
> dstat.

Depending on size and internals of your target HDD, part of the problem
might be that data transfer speed on outer tracks is faster than on
inner tracks.

> It may caused by some other reasons (but rather not a fragmentation - it
> was after an initial test backup performed to an empty disk), but it
> could be also some kind of memory leak in Borg which leads to keeping
> more and more data internally over time causing that slowness.

I don't think there is a memory leak (at least not a significant one).

But when building the in-memory data structures, speed may vary
depending e.g. on size and fill ratio of the hashtables. While doing
that, it uses more and more memory, but that is used memory, not leaked
memory.

You might also observe some property of the python interpreter's memory
manager: it does not like to give back memory to the OS memory
management, but rather keeps it.

> I may try reproduce that behavior later on this month if needed. Borg
> 1.1.8, Fedora 29.

Was it this borg version you used for your measurement?

Long ago there was a bug in borg that led to performance problems in the
hash tables while they filled up (if you want to search for it:
"tombstones").

Also watch whether your RAM is enough. If it uses more than present,
performance goes down the drain due to paging.

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