[MlMt] High Sierra, APFS, Time Machine, and MailMate...

Steven M. Bellovin smb at cs.columbia.edu
Sun Dec 10 21:58:24 EST 2017

On 10 Dec 2017, at 21:31, Charlie Garrison wrote:

> On 11 Dec 2017, at 13:14, Steven M. Bellovin wrote:
>> Seems like an improbable subject line--but is anyone else who uses 
>> MailMate on High Sierra/APFS suddenly having serious Time Machine 
>> performance issues? I am, on two different laptops. An iMac, which 
>> has a hard drive and hence HFS+, is not having any trouble.
> Time Machine is working normally for me.
> You didn’t see what your Time Machine destination is. That would be 
> my first suspect.

I see it to two different external USB disks and to an USB drive 
connected to an Airport. This is two different machines; both show the 
problem going to either of their destinations. On the newer laptop, the 
disk (and the hub it's normally plugged into, via a Thunderbolt dock) is 
USB-3; the older laptop is USB-2 only.
>> The overall symptom is that backups take *many hours*, with sudden, 
>> inexplicable stalls. I say "inexplicable" because Activity Monitor 
>> shows essentially no CPU, network, or disk activity -- but the backup 
>> just *stops*. I normally don't run MailMate on one of the laptops; 
>> its backups complete in a rational amount of time. When I do, it sees 
>> the same stalls. In fact, I'm running MailMate on it right now so 
>> that I can see what happens on my primary laptop when I exit 
>> MailMate. Sure enough, that machine is now behaving.
> Note, TimeMachine takes a snapshot when backing up APFS volumes, so 
> there “shouldn’t” be any difference with MailMate running or 
> not. I suspect that’s really a symptom of what the real problem is.

I was unclear. The issue is not whether MailMate is running during the 
dump; rather, it's whether MailMate has run and therefore received mail 
and modified directories.
>> My suspicion is that the problem has to do with very large 
>> directories on APFS file systems, but I don't know that for sure. I 
>> have some very large mailboxes, though, and these are of course 
>> active when MailMate is running.  And of course, that doesn't explain 
>> why I don't see any system activity.
> Due to the way APFS works, I’d be surprised if that’s the issue.

I don't know the technical details. I guessed at that because it's the 
major difference between the laptop and the iMac.
>> Is anyone else seeing this? Does anyone have any work-arounds, other 
>> than "don't have such large mailboxes" or "don't run APFS"? I do have 
>> a new laptop on order; I'm seriously tempted to reformat it as HFS+ 
>> before I start using it.
> If that new laptop has an SSD, then APFS is your only choice.


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