Hi Qu,
On 2016/03/07 14:42, Qu Wenruo wrote:
Hi,
As many have already known, "btrfs check" is a memory eater.
The problem is, btrfsck checks extent tree in a very comprehensive method.
1) Create extent_ref for each extent item with backref
2) Iterate all other trees to add extent ref
3) If one extent_ref with all ref/backref matches, it's deleted.
The method is good, can found any extent mismatch problem when checking extent tree. (Although it has already iterated the whole fs)
For a large enough filesystem, it may have tegas of extents, and memory is easy eaten up.
We hope to fix it in the following method:
1) Only check extent backref when iterating extent tree
Unlike current implement, we check one extent item and its backref
only.
If one backref can't be reached, then it's an error and output (or
try to fix).
After iterating all backref of an extent item, all related memory is
freed and we won't bother recording anything for later use.
That's to say, we only care backref mismatch case when checking
extent tree.
Case like missing EXTENT_ITEM for some extent is not checked here.
2) Check extent ref while iterating other trees
We only check forward-ref while iterating one tree.
In this step, we only check forward-ref, so we can find the remaining
problem like missing EXTENT_ITEM for given extent.
Any further advice/suggestion? Or is there anyone already doing such work?
Thank you for your effort. I have basic questions.
1. Could you tell me what you'd like to do?
a) Provide completely the same function with current
implementation by other, more efficient way.
b) Replace the current implementation with the quicker
one that provides the limited function.
c) Any other
2. Do you have the estimation that how long does the
new algorithm take compare with the current one?
# Of course, "currently not sure" is OK at this stage.
I'm interested in it because there is the trade-off
between speed and memory consumption in many case,
and btrfsck takes very long time with a large filesystem.
Thanks,
Satoru
Thanks,
Qu
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