[ale] Ubuntu Linux Defrag EXT4

[Greg Freemyer]

For some reason, I just remembered "filefrag".

In this thread I don't recall anyone mentioning it.

If you have a specific file you want to see how it fragmented, then
filefrag is the diagnostic tool for the job.

Greg

On Tue, Sep 14, 2010 at 8:33 AM, Pat Regan <thehead at patshead.com> wrote:
> On Tue, 14 Sep 2010 00:16:45 -0400
> "Michael B. Trausch" <mike at trausch.us> wrote:
>
>> I was using it for awhile, but then I ran into a rather nasty bug that
>> remained unfixed for some time, and I went back to using ext4.
>>
>
> I toyed with it a little back in February or so.  There was no proper
> snapshot removal.  I was toying with the idea of creating snapshots and
> just going the slow route and doing an `rm -rf` to delete snapshots.
> That was fine except btrfs was getting pretty slow as the snapshot
> count increased.
>
> Now I've been running it for root and home for probably close to two
> months now without any problems.  I wonder if I need one of those "No
> file system corruption in X days" posters :).
>
>> However, when I was using it, I was taking advantage of the COW
>> capability of the FS quite a bit.  My daily work was _much_ faster,
>> and the way that I did things changed quite a lot to take advantage
>> of the ability to cheaply create snapshots of individual files and
>> modify them and throw them away if desired.  It is the ultimate
>> filesystem for managing something like a VM on, particularly if that
>> VM runs Windows, because you can do all the snapshoting and moving
>> and deleting and reverting with complete transparency to the VM
>> monitor.  It really is very nice.
>
> At the moment it is the exact opposite of the ultimate file system for
> running virtual machines...  There's some sort of bug/feature when
> running kvm/qemu disk images on top of btrfs.  For me, watching iostat
> in the guest sit in the tens of KB per second on writes was keeping the
> host at a constant steady 1MB+ per second write load...
>
> IIRC, you can eliminate this problem if you turn off barriers.  It
> sounds like turning off barriers pretty much guaranties a corrupt
> file system if the file system isn't unmounted cleanly...
>
> I don't know if the problem is more or less noticeable with spinning
> media, though.
>
> I don't work with large numbers of virtual machines like I used to a
> few years ago (I barely fire any up at all anymore).  I was really
> hoping I could use btrfs snapshots, and deduplication when it is
> available, to reduce the footprint and increase the speed of my virtual
> disks (every shared block is a bit of shared cache).
>
>> I'm looking forward to going back to trying it again; the bugs that I
>> ran into before are (so I hear, anyway) completely fixed now, and I
>> should be able to safely use the filesystem in a multi-device layout
>> as I was previously.  I'm looking forward to the day that btrfs
>> becomes the preferred stable filesystem, myself.
>
> I'm running an awesome and simple little script called `btrfs-snap`.  I
> thought I was going to have to script automated snapshots on my own but
> btrfs-snap is nearly perfect.  I'm keeping 24 hourly, 7 daily, and 4
> weekly snapshots of three volumes on my laptop.
>
> Unfortunately (fortunately?), running btrfs has me even more paranoid
> about running backups...  So I am saving time by getting free instant
> "backups" but I'm spending more time making running rdiff-backup than
> ever...
>
>> I never did get around to trying btrfs' online defragmentation
>> support, but I suspect that I won't need that in majority of the
>> "normal" use cases that I have; fragmentation isn't a problem on my
>> own personal workstations on any other filesystem that I have run on
>> in the past, despite the number of files (and megabytes/gigabytes
>> contained therein) that I have growing significantly in the past
>> several years.  The only real problem that I have with the extX
>> family is that it isn't versatile enough for me, after having had a
>> taste of btrfs.  ;-)
>
> I am imagining that VM disk images stored with COW enabled are going to
> get very fragmented very quickly.
>
> I'm thinking that fragmentation isn't likely to be a huge problem even
> on spinning media.  I imagine it might be worthwhile to derfragment
> database files once every month or quarter or so, maybe...
>
> Pat
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-- 
Greg Freemyer
Head of EDD Tape Extraction and Processing team
Litigation Triage Solutions Specialist
http://www.linkedin.com/in/gregfreemyer
CNN/TruTV Aired Forensic Imaging Demo -
   http://insession.blogs.cnn.com/2010/03/23/how-computer-evidence-gets-retrieved/

The Norcross Group
The Intersection of Evidence & Technology
http://www.norcrossgroup.com