zfs receive stalls whole system

[Ronald Klop]

On Tue, 17 May 2016 01:07:24 +0200, Rainer Duffner  
<rainer at ultra-secure.de> wrote:

> Hi,
>
> I have two servers, that were running FreeBSD 10.1-AMD64 for a long  
> time, one zfs-sending to the other (via zxfer). Both are NFS-servers and  
> MySQL-slaves, the sender is actively used as NFS-server, the recipient  
> is just a warm-standby, in case something serious happens and we don’t  
> want to wait for a day until the restore is back in place. The  
> MySQL-Slaves are actively used as read-only servers (at the application  
> level, Python’s SQL-Alchemy does that, apparently).
>
> They are HP DL380G8 (one CPU, hexacore) with over 128 GB RAM (I think  
> one has 144, the other has 192).
> While they were running 10.1, they used HP P420 RAID-controllers with  
> individual 12 RAID0 volumes that I pooled into 6-disk RAIDZ2 vdevs.
> I use zfsnap to do hourly, daily and weekly snapshots.
>
> Sending worked well, especially after updating to 10.1
>
> Because the storage was over 90% full (and I really hate this  
> RAID0-business we have with the HP RAID controllers), I rebuilt the  
> servers with HPs OEMed H220/221 controllers (LSI 2308 in disguise) and  
> an external disk shelf, hosting 12 additional disks was added- and I  
> upgraded to FreeBSD 10.3.
> Because we didn’t want to throw out the original disks, but increase  
> available space a lot, the new disks are double the size of the original  
> disks (600 vs. 1200 GB SAS).
> I also created GPT-partitions on the disks and labeled them according to  
> the disk’s position in the cages/shelf, created the pools with the  
> got-partition-names instead of the daX-names.
>
> Now, when I do a zxfer, sometimes the whole system stalls while the data  
> is sent over, especially if the delta is large or if something else is  
> reading from the disk at the same time (backup agent).
>
> I had this before, on 10.0 (I believe, we didn’t have this in 9.1  
> either, IIRC) and it went away in 10.1.
>
> It’s very difficult (well, impossible) to debug, because the system  
> totally hangs and doesn’t accept any keypresses.
>
> Would a ZIL help in this case?
> I always thought that NFS was the only thing that did SYNC writes…

Databases love SYNC writes too. (But that doesn't say anything about the  
unresponsive system).
I think there is a statistic somewhere in FreeBSD to analyze the sync vs  
async writes and decide if a ZIL will help or not. (But that doesn't say  
anything about the unresponsive system either).

Ronald.