[lustre-discuss] stripe count recommendation, and proposal for auto-stripe tool
Nathan Dauchy - NOAA Affiliate
nathan.dauchy at noaa.gov
Thu May 19 08:53:35 PDT 2016
Patrick,
You bring up an interesting point on read vs. write performance. We can't
use lfs_migrate control the stripe count used for writes (obviously), so
that is left up to the application developer or at least the user to
intelligently place shared access files in a directory with wider
striping. Restriping a file with lfs_migrate could change *read*
performance characteristics, so there is indeed some risk there... but your
work implies that is not too bad. If we only restripe files that are
"old", then the likelyhood that they will be read again goes way down, and
balancing capacity used plays a bigger factor. Bottom line is that I think
restriping has more potential for upsides than down. :)
Thanks,
Nathan
On Wed, May 18, 2016 at 1:22 PM, Patrick Farrell <paf at cray.com> wrote:
> Nathan,
>
> This *is* excellent fodder for discussion.
>
> A few thoughts from a developer perspective. When you stripe a file to
> multiple OSTs, you're spreading the data out across multiple targets, which
> (to my mind) has two purposes:
> 1) More even space usage across OSTs (mostly relevant for *really* big
> files, since in general, singly striped files are distributed across OSTs
> anyway)
> 2) Better bandwidth/parallelism for accesses to the file.
>
> The first one lends itself well to a file size based heuristic, but I'm
> not sure the second one does. That's more about access patterns. I'm not
> sure that you see much bandwidth benefit from striping with a single
> client, at least as long as an individual OST is fast relative to a client
> (increasingly common, I think, with flash and larger RAID arrays). So
> then, whatever the file size, if it's accessed from one client, it should
> probably be single striped.
>
> Also, for shared files, client count relative to stripe count has a huge
> impact on write performance. Assuming strided I/O patterns, anything more
> than 1 client per stripe/OST is actually worse than 1 client. (See my lock
> ahead presentation at LUG'15 for more on this.) Read performance doesn't
> share this weirdness, though.
>
> All that's to say that for case 2 above, at least for writing, it's access
> pattern/access parallelism, not size, which matters. I'm sure there's some
> correlation between file size and how parallel the access pattern is, but
> it might be very loose, and at least write performance doesn't scale
> linearly with stripe size. Instead, the behavior is complex.
>
> So in order to pick an ideal striping with case 2 in mind, you really need
> to understand the application access pattern. I can't see another way to
> do that goal justice. (The Lustre ADIO in the MPI I/O library does this,
> partly by controlling the I/O pattern through I/O aggregation for
> collective I/Os.)
>
> So I think your tool can definitely help with case 1, not so sure about
> case 2.
>
> - Patrick
>
> On 05/18/2016 12:22 PM, Nathan Dauchy - NOAA Affiliate wrote:
>
> Greetings All,
>
> I'm looking for your experience and perhaps some lively discussion
> regarding "best practices" for choosing a file stripe count. The Lustre
> manual has good tips on "Choosing a Stripe Size", and in practice the
> default 1M rarely causes problems on our systems. Stripe Count on the other
> hand is far more difficult to chose a single value that is efficient for a
> general purpose and multi-use site-wide file system.
>
> Since there is the "increased overhead" of striping, and weather
> applications do unfortunately write MANY tiny files, we usually keep the
> filesystem default stripe count at 1. Unfortunately, there are several
> users who then write very large and shared-access files with that default.
> I would like to be able to tell them to restripe... but without digging
> into the specific application and access pattern it is hard to know what
> count to recommend. Plus there is the "stripe these but not those"
> confusion... it is common for users to have a few very large data files and
> many small log or output image files in the SAME directory.
>
> What do you all recommend as a reasonable rule of thumb that works for
> "most" user's needs, where stripe count can be determined based only on
> static data attributes (such as file size)? I have heard a "stripe per GB"
> idea, but some have said that escalates to too many stripes too fast. ORNL
> has a knowledge base article that says use a stripe count of "File size /
> 100 GB", but does that make sense for smaller, non-DOE sites? Would stripe
> count = Log2(size_in_GB)+1 be more generally reasonable? For a 1 TB file,
> that actually works out to be similar to ORNL, only gets there more
> gradually:
> https://www.olcf.ornl.gov/kb_articles/lustre-basics/#Stripe_Count
>
> Ideally, I would like to have a tool to give the users and say "go
> restripe your directory with this command" and it will do the right thing
> in 90% of cases. See the rough patch to lfs_migrate (included below) which
> should help explain what I'm thinking. Probably there are more efficient
> ways of doing things, but I have tested it lightly and it works as a
> proof-of-concept.
>
> With a good programmatic rule of thumb, we (as a Lustre community!) can
> eventually work with application developers to embed the stripe count
> selection into their code and get things at least closer to right up
> front. Even if trial and error is involved to find the optimal setting, at
> least the rule of thumb can be a _starting_point_ for the users, and they
> can tweak it from there based on application, model, scale, dataset, etc.
>
> Thinking farther down the road, with progressive file layout, what
> algorithm will be used as the default? If Lustre gets to the point where
> it can rebalance OST capacity behind the scenes, could it also make some
> intelligent choice about restriping very large files to spread out load and
> better balance capacity? (Would that mean we need a bit set on the file to
> flag whether the stripe info was set specifically by the user or
> automatically by Lustre tools or it was just using the system default?)
> Can the filesystem track concurrent access to a file, and perhaps migrate
> the file and adjust stripe count based on number of active clients?
>
> I appreciate any and all suggestions, clarifying questions, heckles, etc.
> I know this is a lot of questions, and I certainly don't expect definitive
> answers on all of them, but I hope it is at least food for thought and
> discussion! :)
>
> Thanks,
> Nathan
>
>
> --- lfs_migrate-2.7.1 2016-05-13 12:46:06.828032000 +0000
> +++ lfs_migrate.auto-count 2016-05-17 21:37:19.036589000 +0000
> @@ -21,8 +21,10 @@
>
> usage() {
> cat -- <<USAGE 1>&2
> -usage: lfs_migrate [-c <stripe_count>] [-h] [-l] [-n] [-q] [-R] [-s] [-y]
> [-0]
> +usage: lfs_migrate [-A] [-c <stripe_count>] [-h] [-l] [-n] [-q] [-R] [-s]
> [-v] [-y] [-0]
> [file|dir ...]
> + -A restripe file using an automatically selected stripe count
> + currently Stripe Count = Log2(size_in_GB)
> -c <stripe_count>
> restripe file using the specified stripe count
> -h show this usage message
> @@ -31,11 +33,11 @@
> -q run quietly (don't print filenames or status)
> -R restripe file using default directory striping
> -s skip file data comparison after migrate
> + -v be verbose and print information about each file
> -y answer 'y' to usage question
> -0 input file names on stdin are separated by a null character
>
> -The -c <stripe_count> option may not be specified at the same time as
> -the -R option.
> +Only one of the '-A', '-c', or '-R' options may be specified at a time.
>
> If a directory is an argument, all files in the directory are migrated.
> If no file/directory is given, the file list is read from standard input.
> @@ -48,15 +50,19 @@
>
> OPT_CHECK=y
> OPT_STRIPE_COUNT=""
> +OPT_AUTOSTRIPE=""
> +OPT_VERBOSE=""
>
> -while getopts "c:hlnqRsy0" opt $*; do
> +while getopts "Ac:hlnqRsvy0" opt $*; do
> case $opt in
> + A) OPT_AUTOSTRIPE=y;;
> c) OPT_STRIPE_COUNT=$OPTARG;;
> l) OPT_NLINK=y;;
> n) OPT_DRYRUN=n; OPT_YES=y;;
> q) ECHO=:;;
> R) OPT_RESTRIPE=y;;
> s) OPT_CHECK="";;
> + v) OPT_VERBOSE=y;;
> y) OPT_YES=y;;
> 0) OPT_NULL=y;;
> h|\?) usage;;
> @@ -69,6 +75,16 @@
> echo "$(basename $0) error: The -c <stripe_count> option may not" 1>&2
> echo "be specified at the same time as the -R option." 1>&2
> exit 1
> +elif [ "$OPT_STRIPE_COUNT" -a "$OPT_AUTOSTRIPE" ]; then
> + echo ""
> + echo "$(basename $0) error: The -c <stripe_count> option may not" 1>&2
> + echo "be specified at the same time as the -A option." 1>&2
> + exit 1
> +elif [ "$OPT_AUTOSTRIPE" -a "$OPT_RESTRIPE" ]; then
> + echo ""
> + echo "$(basename $0) error: The -A option may not be specified at" 1>&2
> + echo "the same time as the -R option." 1>&2
> + exit 1
> fi
>
> if [ -z "$OPT_YES" ]; then
> @@ -107,7 +123,7 @@
> $ECHO -n "$OLDNAME: "
>
> # avoid duplicate stat if possible
> - TYPE_LINK=($(LANG=C stat -c "%h %F" "$OLDNAME" || true))
> + TYPE_LINK=($(LANG=C stat -c "%h %F %s" "$OLDNAME" || true))
>
> # skip non-regular files, since they don't have any objects
> # and there is no point in trying to migrate them.
> @@ -127,11 +143,6 @@
> continue
> fi
>
> - if [ "$OPT_DRYRUN" ]; then
> - echo -e "dry run, skipped"
> - continue
> - fi
> -
> if [ "$OPT_RESTRIPE" ]; then
> UNLINK=""
> else
> @@ -140,16 +151,43 @@
> # then we don't need to do this getstripe/mktemp stuff.
> UNLINK="-u"
>
> - [ "$OPT_STRIPE_COUNT" ] && COUNT=$OPT_STRIPE_COUNT ||
> - COUNT=$($LFS getstripe -c "$OLDNAME" \
> - 2> /dev/null)
> SIZE=$($LFS getstripe $LFS_SIZE_OPT "$OLDNAME" \
> 2> /dev/null)
> + if [ "$OPT_AUTOSTRIPE" ]; then
> + FILE_SIZE=${TYPE_LINK[3]}
> + # (math in bash is dumb, so depend on common tools, and there are
> options for that...)
> + # Stripe Count = Log2(size_in_GB)
> + #COUNT=$(echo $FILE_SIZE | awk '{printf
> "%.0f\n",log($1/1024/1024/1024)/log(2)}')
> + #COUNT=$(printf "%.0f\n" $(echo "l($FILE_SIZE/1024/1024/1024) / l(2)" |
> bc -l))
> + COUNT=$(echo "l($FILE_SIZE/1024/1024/1024) / l(2) + 1" | bc -l | cut -d
> . -f 1)
> + # Stripe Count = size_in_GB
> + #COUNT=$(echo "scale=0; $FILE_SIZE/1024/1024/1024" | bc -l | cut -d .
> -f 1)
> + [ "$COUNT" -lt 1 ] && COUNT=1
> + # (does it make sense to skip the file if old
> + # and new stripe count are identical?)
> + else
> + [ "$OPT_STRIPE_COUNT" ] && COUNT=$OPT_STRIPE_COUNT ||
> + COUNT=$($LFS getstripe -c "$OLDNAME" \
> + 2> /dev/null)
> + fi
>
> [ -z "$COUNT" -o -z "$SIZE" ] && UNLINK=""
> - SIZE=${LFS_SIZE_OPT}${SIZE}
> fi
>
> + if [ "$OPT_DRYRUN" ]; then
> + if [ "$OPT_VERBOSE" ]; then
> + echo -e "dry run, would use count=${COUNT} size=${SIZE}"
> + else
> + echo -e "dry run, skipped"
> + fi
> + continue
> + fi
> + if [ "$OPT_VERBOSE" ]; then
> + echo -n "(count=${COUNT} size=${SIZE}) "
> + fi
> +
> + [ "$SIZE" ] && SIZE=${LFS_SIZE_OPT}${SIZE}
> +
> # first try to migrate inside lustre
> # if failed go back to old rsync mode
> if [[ $RSYNC_MODE == false ]]; then
>
>
>
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