[Users] Zimbra 8.8 Swap File Usage

Victor d'Agostino d.agostino.victor at gmail.com
Thu Jan 17 08:00:12 CET 2019


I too have IO and swap issues on 8.8.8 release when users delete a lot of
email because Zimbra does not implement the IMAP "move" fonction. It's not
mandatory because Zimbra uses symlink so copying and deleting rather than
moving is fine.

BUT I found out in August that when zextras powerstore module is enabled
Zimbra does not use the symlink system anymore ! Another java class is used.

After disabling the powerstore module the IO load was lowered about 20 %.

Zextras is ahead of this issue, but I dont know if a fix was released.


Le jeu. 17 janv. 2019 à 00:00, L Mark Stone <lmstone at lmstone.com> a écrit :

> If you are running Zimbra 8.8 and seeing unexpected swap file usage, I
> have a suggestion you may want to try.
> After doing a number of migrations of older Zimbra systems that never
> touched the swap file, I started seeing that these systems were now using
> the swap file a fair amount -- even with vm.swappiness set to 0.  A 16GB
> server with under 400 mailboxes after just a few minutes would start using
> the swap file, and after a day or so would be using several GB of swap.
> At first I suspected something in Zimbra/ZeXtras, but after I rebooted two
> small servers and increased their instance sizes to go from 16GB of RAM to
> 64GB of RAM, and they too started swapping within a few minutes, I felt
> this wasn't likely to be a Zimbra/ZeXtras issue.  Even when these systems
> had 16GB RAM and were using a lot of swap, none of the users were
> complaining about reduced performance.  The systems remained quite snappy
> at both RAM inventories.
> After a week or so of research, last night I ran a test on few systems,
> and this morning those systems' swap file usage is about half or less of
> what it has historically been.
> So I wanted to share what I've done, and ask for others who know more
> about this to provide feedback as to whether what I've done is
> optimal/appropriate -- or not.
> By way of background, modern distros/kernels now make greater use of what
> top reports as "buff(ers)/cache".  /var/run for example now is a symlink to
> /run on tmpfs, and this lives in that memory space.  If you run a ram disk
> for Amavis's tmp directory as I do, that memory also as I understand it is
> living in what top reports as buff/cache.
> But on the Zimbra systems that were swapping heavily, buff/cache was
> unexpectedly large.  Even a 32GB server with under 500 users would report
> 20GB or more as buff/cache, and maybe 5-10GB of swap file usage.  A
> 500-mailbox server should easily be performant with 12GB-16GB of RAM.
> What I came across was /proc/sys/vm/vfs_cache_pressure
> This setting, which ranges from 0 to 200 and has a default of 100,
> controls how much "pressure" is put on the kernel to release memory from
> the buff/cache pools.  The higher the number, the more aggressively the
> kernel is "pressed" to release memory from the buff/cache pools.
> This to me seems not so dissimilar from vm.swappiness, which controls how
> aggressively the kernel is "pressed" to move data out of RAM and into
> swap.  The default there is 60 (lots of swap file usage) and Zimbra's best
> practice is to set this to zero.
> So last night I ran as root "sysctl -w vm.vfs_cache_pressure=150" on a
> few systems, followed by "swapoff -a && swapon -a"
> This morning, swap file usage was cut in at least half of what it is has
> been of late on all the systems on which I have made this change.  More
> importantly, end users continue to report the systems remain highly
> performant.
> Anecdotally, I've noticed on one system that an IMAPSYNC that had been
> running for a few days, and which had averaged ~3.75-4.0 messages per
> second through most mailboxes, has been running this morning through the
> remaining mailboxes at more than 5 messages per second.
> I would be grateful if your Zimbra system(s) are using the swap file more
> heavily than expected, if you wouldn't mind trying this adjustment and
> report back.  If you have experience with this setting either with Zimbra
> or other applications, please share -- especially if you feel, like setting
> vm.swappiness to zero, we should set vfs_cache_pressure to its max value
> of 200.
> On next week's Zeta Alliance call I intend to add this to the agenda (and
> if you'd like an invite to that call, send me an email to
> mark.stone at MissionCriticalEmail.com).  So if you are seeing unexpected
> swap file usage and want to try this, you can share your results on the
> call next week and/or on this list.
> Thanks,
> Mark
> *_________________________________________________*
> *Another Message From...   L. Mark Stone*
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