Very long delay for first write to big filesystem

[Ken Bass]

Thx Andreas.

re: block bitmaps - yes that is what I really meant. My experience with
filesystems is mainly from CPM/BDOS, where "directory" and block mapping
are essentially synonymous.

And now I understand about the timing. Makes sense when you describe it
that way.

My system is ext4, although I doubt that I used "flex_bg" option, since
this was first created awhile back. I did try to run e4defrag. It simply
said that no defrag was needed.

So, now I'm only left in  need of a work-around. Perhaps a way to have the
system load the bitmaps at boot time in the background? It would need to be
done in such a way that it would not block any other access to that system.
Or, is there a better filesystem format that would not have this problem?
(Not a really great solution, since I would need to somehow/somewhere
backup my 7.5TB system first.)

It does seem strange that this hasn't become a more serious issue, as
typical filesystems are getting bigger now. And I can't imagine a really
large network server (10TB+) having to deal with this.

Again, thx for the response.

ken


On Sat, Jan 18, 2014 at 9:09 AM, Andreas Dilger <adilger at dilger.ca> wrote:

> On Jan 17, 2014, at 9:32, Ken Bass <daytooner at gmail.com> wrote:
> >
> > The problem/issue: there is a very long delay when my system does a
> write to the filesystem. The delay now is over 5 minutes (yes: minutes).
> This only happens on the first write after booting up the system, and only
> for large files - 1GB or more. This can be a serious problem since all
> access to any hard disk is blocked and will hang until the first write
> begins again.
> >
> > The prevailing thought at the time was this was associated with loading
> into memory the directory information looking for free space, which I would
> believe now.
>
> It isn't actually directory information that is being loaded, but rather
> the
> block bitmaps from each group, and each one needs a seek to read.
> This will take up to 7.5 TB / 128 MB/group / 100 seeks/sec = 600s
> if the filesystem is nearly full. After this point, the bitmaps are cached
> In memory and allocation is faster.
>
> > The filesystem in question is 7.5TB, with about 4TB used. There are over
> 250,000 files. I also have another system with 1TB total and 400GB used,
> with 65,000 files. This system, the smaller one, is beginning to show
> delays as well, although only a few seconds.
> >
> > This problem seems to involve several factors: the total size of the
> system; the current "fragmentation" of that system; and finally the amount
> of physical memory available.
> >
> > As to the last factor, the 7.5TB system has only 2GB of memory (I didn't
> think that it would need a lot since it is mostly being used as a file
> server). The "fragmentation" factor (I am only guessing here) occurs with
> having many files written and deleted over time.
> >
> > So my questions are: is there a solution or work around for this; and is
> this a bug, or perhaps an undesirable feature. If the latter, should this
> be reported (somewhere)?
>
> You might consider mounting the filesystem as ext4 instead of ext3.
> It will do a slightly better job of finding contiguous free space
> and avoid loading bitmaps that do not have enough space, but the
> physics of seeking to read bitmaps is still the same.
>
> If you format a new filesystem as ext4 (as opposed to just mounting the
> existing filesystem as ext4) you can use a new feature "flex_bg" that
> locates the block and inode bitmaps together so that they can be read
> without so much seeking. You'd need a spare disk to format and copy
> the data over to.
>
> Using ext4 is also more resistant to fragmentation over time.
>
> Cheers, Andreas
>
> > Any suggestions, tips, etc. greatly appreciated.
> >
> > TIA
> >
> > ken
> >
> > _______________________________________________
> > Ext3-users mailing list
> > Ext3-users at redhat.com
> > https://www.redhat.com/mailman/listinfo/ext3-users
>
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