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Re: Very long delay for first write to big filesystem



On Jan 19, 2014, at 7:07 PM, Ken Bass <daytooner gmail com> wrote:
> 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.

Use "dumpe2fs -h /dev/XXX | grep feature" to see if it is listed.

> 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.

We had a similar problem in the past.  Run "dumpe2fs /dev/XXX > /dev/null"
at startup time (can be before or after mount) to start it reading the
block and inode allocation bitmaps.

>  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.)

Yes, formatting with "mke2fs -t ext4" should enable flex_bg by default.

> 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.

That's why the flex_bg feature was added to ext4 in the first place.

Cheers, Andreas

> Again, thx for the response.
> 
> ken
> 
> 
> On Sat, Jan 18, 2014 at 9:09 AM, Andreas Dilger <adilger dilger ca> wrote:
> On Jan 17, 2014, at 9:32, Ken Bass <daytooner 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
> >
> > _______________________________________________
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> > Ext3-users redhat com
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> 
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Cheers, Andreas





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