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Tag: ufs2
EuroBSDCon 2008 - Nick Barkas - Dynamic memory allocation for dirhash in UFS2
Source: EuroBSDCon
Added: 22 October 2008
Tags: eurobsdcon, eurobsdcon2008, ufs2, nick barkas
MP3 (1 byte, 32 minutes), OGG (1 byte, 32 minutes), PDF (1 byte, n pages)
Hello My name is Nick Barkas. I'm a master's student
studying scientific computing at Kungliga Tekniska
hgskolan (KTH) in Stockholm, Sweden. I have just
begun work on a Google Summer of Code project with
FreeBSD: Dynamic memory allocation for dirhash in
UFS2 . I would like to present my results from
this project at EuroBSDCon this year. This project
is very much a work in progress now so it is a bit
difficult to summarize what I would ultimately
present. I will try to describe an outline, though.
First I will give background information on dirhash:
an explanation of the directory data structure in
UFS2, how directory lookups in this structure
necessitate a linear search, and how dirhash speeds
these lookups up without having to change anything
about the directory data structure. Next I will
explain the current limitation that dirhash's maximum
memory use must be manually specified by administrators,
or left at a small conservative default of 2MB. I
will explain some different methods I will have
explored to try and make this maximum memory limit
dynamically increase and decrease as the system has
more or less free memory, and which method I will
have ultimately settled on and implemented. Then
I'll present some test results of performance of
operations on very large directories with and without
dynamic memory allocation enabled for dirhash. Next
I will talk about how speed gains from dirhash are
limited by the fact that the hash tables exist only
in memory and must be recreated after each system
boot, as big directories are scanned for the first
time, or even have to be recreated for a directory
that has not been scanned in some time if its dirhash
has been discarded to free memory. These problems
can be eliminated by using an on-disk index for
directory entries. I will talk about some of the
challenges of implementing on-disk indexing, such
as remaining backwards compatible with older versions
of UFS2 and interoperating properly with softupdates.
Then, if my SoC project has permitted me time to
work on this aspect of it, I will explain some
possible methods for adding directory indexing to
UFS2 that meets these challenges, and which of those
ideas I will have implemented. Finally I will present
results of some benchmarks on this filesystem with
indices, and compare to performance with dirhash,
and with no indices or dirhashes.
Keywords: dirhash, ufs2, filesystems, performance tuning
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