[zfs-discuss] Re: Copying a file from one ZFS to another starves all other processes

RangerRick rbraddy at gmail.com
Sun May 19 11:36:35 EDT 2013

Hi Ryan,

What version of Linux are you running?  We have many hundreds of customers 
using ZOL with SoftNAS and have had no such issues with ZOL on CentOS 6.x.

However, I have seen something similar to what you describe occur when 
files are copied to and from the LOCAL filesystem (non-ZFS) only on a ZOL 
system.  Local filesystem caching appears to put pressure on available 
memory, which in turn causes what appears to be a lockup until that memory 
pressure is relieved.  We have not observed this when the files being 
copied are ZFS to ZFS - only if done on the root filesystem, which is 
non-ZFS.  As we do not use the root filesystem for file sharing, we do not 
encounter this in practice.


On Friday, May 17, 2013 11:29:32 PM UTC-5, Ryan How wrote:
> With all the ZFS problems I'm having, it seems that it must be a problem 
> with my kernel or something, coz otherwise I'm sure there would be a lot 
> more people posting issues on the mailing list and a lot less people 
> using ZOL! 
> They all almost seem related to I/O scheduling and buffering (well the 
> symptoms of it, not sure what the actual problem is) 
> One of the problems I'm having is when copying a file (cp fileA fileB), 
> it basically starves everything else. Then my VMs (running as file 
> backed storage directly on ZFS) get corrupt filesystems reporting that 
> the disk controller was not responding (I'm guessing because they are 
> trying to write, get to response from the system, so the guest things 
> the virtual disk is faulty) 
> Does anyone else get anything like this? 
> When copying as well, all the zfs commands are slow to respond (eg. 
> zpool iostat takes a few seconds to spit out any info) 
> Not using dedup or compression, machine has 32GB ram of which 6GB is 
> currently free (after running for a few hours) 
> I'm not really fussed if performance is slow (but performance is 
> actually quite good), but if I am doing more than 1 disk related thing 
> at a time it is kinda useless. 
> Thanks, Ryan 
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