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[Linux-cluster] Locking and performance questions regarding GFS1/2



Hello GFS developpers,

I have a few questions regarding how locking is performed in GFS, and
the improvements brought by GFS2.

When I perform "ls" on a root directory of GFS1 that's been freshly
mounted, it takes a time linear to the size of the FS. Nevertheless, it
appears that the number of locks taken by GFS is always the same.
When i perform this a second time, the command returns almost directly.
What's the problem ? Was it solved in GFS2 ?

When I perform "mkdir" or "touch" on either the root of a freshly
mounted GFS1, or either on a subdirectory, it takes a time linear to
the size of the FS. I understand that it must determine the best RG to
put the dinode into, therefore reading a number of RG linear to the
size of the FS (if I don't play with the RG size), and taking a number
of locks also linear to the size of the FS.... This is the same
behaviour as when I perform "df", i guess.
Is this behaviour different in GFS2 ? Wouldn't be a possibility for
better behaviours, like, for example, taking the first free RG, if
we encounter such a RG (which is the case when the FS has just
been formated) ? Or maintaining fuzzy data about RG in an inode
(just like it is done for the fuzzy statfs)  ? Or maybe this is useless,
since it happens only at the first time after the FS is mounted on the
first node, and you consider that a FS is not mounted/unmounted
frequently ?
However, has this been changed in GFS2 ?

When i read this:
http://sourceware.org/cluster/faq.html#gfs_tuning
I understand that i should increase the size of the RG on big FS.
However, the code says that some data structures are loaded in memory
for each RG that's being locked (notably 2 bitmaps). So there's a
memory overhead when I increase the size of the RG. I also understand
that increasing the size of the RG increases the risk to have 2 or more
nodes working in the same RGs (is this right ?). What is the maximum
size of RG I should be using ?

More generally, is there the list of hard points in GFS1 that you've
been trying to solve with GFS2, somewhere accessible on the web ? Also,
what is actually the maximum size that GFS2 is known to be working on ?
(both in terms of nodes and real size)

Thanks for reading this (too long) list of questions :-)

--
Mathieu


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