* Slides from Ceph talk at linux.conf.au?
@ 2010-02-02 21:05 Craig Dunwoody
2010-02-02 21:43 ` Sage Weil
0 siblings, 1 reply; 6+ messages in thread
From: Craig Dunwoody @ 2010-02-02 21:05 UTC (permalink / raw)
To: ceph-devel; +Cc: cdunwoody
Hello Sage,
Could you make available the slides from your recent Ceph talk at
linux.conf.au? I'd be very interested to see them, and I expect that
others here would as well.
Craig Dunwoody
GraphStream Incorporated
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* Re: Slides from Ceph talk at linux.conf.au?
2010-02-02 21:05 Slides from Ceph talk at linux.conf.au? Craig Dunwoody
@ 2010-02-02 21:43 ` Sage Weil
2010-02-04 11:54 ` Craig Dunwoody
2010-02-05 0:17 ` Hard links Chris Dunlop
0 siblings, 2 replies; 6+ messages in thread
From: Sage Weil @ 2010-02-02 21:43 UTC (permalink / raw)
To: Craig Dunwoody; +Cc: ceph-devel
Hi Craig,
On Tue, 2 Feb 2010, Craig Dunwoody wrote:
> Hello Sage,
>
> Could you make available the slides from your recent Ceph talk at
> linux.conf.au? I'd be very interested to see them, and I expect that
> others here would as well.
I've posted the openoffice presentation at
http://ceph.newdream.net/presentations/
There's also a PDF version, although some slides may look weird due to the
animated figures.
I think LCA should have the slides (and video) up soon for all the talks
as well.
Enjoy!
sage
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* Re: Slides from Ceph talk at linux.conf.au?
2010-02-02 21:43 ` Sage Weil
@ 2010-02-04 11:54 ` Craig Dunwoody
2010-02-05 0:17 ` Hard links Chris Dunlop
1 sibling, 0 replies; 6+ messages in thread
From: Craig Dunwoody @ 2010-02-04 11:54 UTC (permalink / raw)
To: Sage Weil; +Cc: cdunwoody, ceph-devel
Hi Sage,
sage writes:
>I've posted the openoffice presentation at
>http://ceph.newdream.net/presentations/
>There's also a PDF version, although some slides may look weird due to the
>animated figures.
>I think LCA should have the slides (and video) up soon for all the talks
>as well.
Great presentation, thanks for posting this!
Craig Dunwoody
GraphStream Incorporated
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* Hard links
2010-02-02 21:43 ` Sage Weil
2010-02-04 11:54 ` Craig Dunwoody
@ 2010-02-05 0:17 ` Chris Dunlop
2010-02-05 20:34 ` Sage Weil
1 sibling, 1 reply; 6+ messages in thread
From: Chris Dunlop @ 2010-02-05 0:17 UTC (permalink / raw)
To: ceph-devel
G'day Sage,
Sage Weil <sage <at> newdream.net> writes:
> I've posted the openoffice presentation at
>
> http://ceph.newdream.net/presentations/
The last slide (39) mentions "Hard links are rare!".
This isn't necessarily true in a backup system where each
snapshot hard links to the previous snapshot for files that
haven't changed, e.g. an 'rsnapshot' installation.
For some hard numbers, one of our server backups has 72316 files
in yesterday's set, with only 194 not hard linked, and 62141
have 76 hard links (there are currently 76 days of backups for
this server). This is one of 66 servers being backed up to this
one 4.5 TB storage pool.
Does the "hard links are rare" assertion imply that ceph may
have some issues (e.g. hard limits or performance) handling very
large numbers of hard links?
E.g. I see that hard links are mentioned in your Dec 2007
dissertation on ceph, along with the use of an anchor table
which is "managed by a single MDS". Might this be an issue
(e.g. a 'hot spot') for situations with a large number of hard
links such as that described above?
Cheers,
Chris
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* Re: Hard links
2010-02-05 0:17 ` Hard links Chris Dunlop
@ 2010-02-05 20:34 ` Sage Weil
2010-02-07 1:09 ` Chris Dunlop
0 siblings, 1 reply; 6+ messages in thread
From: Sage Weil @ 2010-02-05 20:34 UTC (permalink / raw)
To: Chris Dunlop; +Cc: ceph-devel
On Fri, 5 Feb 2010, Chris Dunlop wrote:
> G'day Sage,
>
> Sage Weil <sage <at> newdream.net> writes:
> > I've posted the openoffice presentation at
> >
> > http://ceph.newdream.net/presentations/
>
> The last slide (39) mentions "Hard links are rare!".
>
> This isn't necessarily true in a backup system where each
> snapshot hard links to the previous snapshot for files that
> haven't changed, e.g. an 'rsnapshot' installation.
>
> For some hard numbers, one of our server backups has 72316 files
> in yesterday's set, with only 194 not hard linked, and 62141
> have 76 hard links (there are currently 76 days of backups for
> this server). This is one of 66 servers being backed up to this
> one 4.5 TB storage pool.
>
> Does the "hard links are rare" assertion imply that ceph may
> have some issues (e.g. hard limits or performance) handling very
> large numbers of hard links?
>
> E.g. I see that hard links are mentioned in your Dec 2007
> dissertation on ceph, along with the use of an anchor table
> which is "managed by a single MDS". Might this be an issue
> (e.g. a 'hot spot') for situations with a large number of hard
> links such as that described above?
Yes and no. The performance impact of hard links is low for the
common backup scenario, but the anchor table scaling has not been
address (it's still a single MDS).
What slide 39 doesn't include is a description of the figure. One of the
most common use scenarios of hard links is what I called 'parallel' links,
where many files in one directory are all hard linked to parallel files in
a different directory, which is exactly what you see with cp -al or
rsnapshot. In that case, the cost of doing a lookup in the anchor
table is amortized over the whole directory.
The anchor table is still maintained by a single MDS, though, and it's all
in RAM at once, so it will be a scaling problem if the fs has a lot of
hard links. That just needs some design attention at some point.
sage
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* Re: Hard links
2010-02-05 20:34 ` Sage Weil
@ 2010-02-07 1:09 ` Chris Dunlop
0 siblings, 0 replies; 6+ messages in thread
From: Chris Dunlop @ 2010-02-07 1:09 UTC (permalink / raw)
To: ceph-devel
Sage Weil <sage <at> newdream.net> writes:
> On Fri, 5 Feb 2010, Chris Dunlop wrote:
>> Does the "hard links are rare" assertion imply that ceph may
>> have some issues (e.g. hard limits or performance) handling very
>> large numbers of hard links?
>>
>> E.g. I see that hard links are mentioned in your Dec 2007
>> dissertation on ceph, along with the use of an anchor table
>> which is "managed by a single MDS". Might this be an issue
>> (e.g. a 'hot spot') for situations with a large number of hard
>> links such as that described above?
>
> Yes and no. The performance impact of hard links is low for the
> common backup scenario, but the anchor table scaling has not been
> address (it's still a single MDS).
>
> What slide 39 doesn't include is a description of the figure. One of the
> most common use scenarios of hard links is what I called 'parallel' links,
> where many files in one directory are all hard linked to parallel files in
> a different directory, which is exactly what you see with cp -al or
> rsnapshot. In that case, the cost of doing a lookup in the anchor
> table is amortized over the whole directory.
>
> The anchor table is still maintained by a single MDS, though, and it's all
> in RAM at once, so it will be a scaling problem if the fs has a lot of
> hard links. That just needs some design attention at some point.
"Just a small matter of designing and coding..." :-)
With the current design of the anchor table maintained by a
single MDS, does this have an impact on the resiliancy of the
system? Specifically, what happens if that one MDS becomes
unavailable?
How much memory does the anchor table require? E.g. for a
million, or 100 million hard links?
Chris
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end of thread, other threads:[~2010-02-07 1:09 UTC | newest]
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-- links below jump to the message on this page --
2010-02-02 21:05 Slides from Ceph talk at linux.conf.au? Craig Dunwoody
2010-02-02 21:43 ` Sage Weil
2010-02-04 11:54 ` Craig Dunwoody
2010-02-05 0:17 ` Hard links Chris Dunlop
2010-02-05 20:34 ` Sage Weil
2010-02-07 1:09 ` Chris Dunlop
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