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<div>Cheers, Andreas</div>
<div>-- </div>
<div>Andreas Dilger</div>
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<div>Lustre Principal Architect</div>
<div>Intel High Performance Data Division</div>
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<div>On 2016/05/18, 11:22, "Nathan Dauchy - NOAA Affiliate" <<a href="mailto:nathan.dauchy@noaa.gov">nathan.dauchy@noaa.gov</a>> wrote:</div>
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<div>Greetings All,</div>
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<div>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.</div>
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<div>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.</div>
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<div>This is exactly what the ORNL "Progressive File Layout" (PFL) project is about. Automatically increase the stripe size of a file as the size grows. That will allow a single default layout to describe both small and large files, and go from e.g. 1 stripe
to 8 stripes to 256 stripes as the size increases.</div>
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<div>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:</div>
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<div> <a href="https://www.olcf.ornl.gov/kb_articles/lustre-basics/#Stripe_Count" target="_blank">
https://www.olcf.ornl.gov/kb_articles/lustre-basics/#Stripe_Count</a></div>
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<div>Using the log2() value seems reasonable.</div>
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<div>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.</div>
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<div>I'd welcome this as a patch submitted to Gerrit.</div>
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<div>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.</div>
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<div>Thinking farther down the road, with progressive file layout, what algorithm will be used as the default?</div>
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<div>To be clear, the PFL implementation does not currently have an algorithmic layout, rather a series of thresholds based on file size that will select different layouts (initially stripe counts, but could be anything including stripe size, OST pools, etc).
The PFL size thresholds and stripe counts _could_ be set up (manually) as as a geometric series, but they can also be totally arbitrary if you want.</div>
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<div>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?</div>
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<div>I think this would be an interesting task for RobinHood, since it already has much of this information. It could find large files with low stripe counts and restripe them during OST rebalancing.</div>
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<div>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! :)</div>
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<div>One last comment on the patch below:</div>
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<div>--- lfs_migrate-2.7.1<span style="white-space: pre-wrap;"> </span>2016-05-13 12:46:06.828032000 +0000</div>
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<div>+++ lfs_migrate.auto-count<span style="white-space:pre-wrap"> </span>2016-05-17 21:37:19.036589000 +0000</div>
<div>@@ -21,8 +21,10 @@</div>
<div> </div>
<div> usage() {</div>
<div> cat -- <<USAGE 1>&2</div>
<div>-usage: lfs_migrate [-c <stripe_count>] [-h] [-l] [-n] [-q] [-R] [-s] [-y] [-0]</div>
<div>+usage: lfs_migrate [-A] [-c <stripe_count>] [-h] [-l] [-n] [-q] [-R] [-s] [-v] [-y] [-0]</div>
<div> [file|dir ...]</div>
<div>+ -A restripe file using an automatically selected stripe count</div>
<div>+ currently Stripe Count = Log2(size_in_GB)</div>
<div> -c <stripe_count></div>
<div> restripe file using the specified stripe count</div>
<div> -h show this usage message</div>
<div>@@ -31,11 +33,11 @@</div>
<div> -q run quietly (don't print filenames or status)</div>
<div> -R restripe file using default directory striping</div>
<div> -s skip file data comparison after migrate</div>
<div>+ -v be verbose and print information about each file</div>
<div> -y answer 'y' to usage question</div>
<div> -0 input file names on stdin are separated by a null character</div>
<div> </div>
<div>-The -c <stripe_count> option may not be specified at the same time as</div>
<div>-the -R option.</div>
<div>+Only one of the '-A', '-c', or '-R' options may be specified at a time.</div>
<div> </div>
<div> If a directory is an argument, all files in the directory are migrated.</div>
<div> If no file/directory is given, the file list is read from standard input.</div>
<div>@@ -48,15 +50,19 @@</div>
<div> </div>
<div> OPT_CHECK=y</div>
<div> OPT_STRIPE_COUNT=""</div>
<div>+OPT_AUTOSTRIPE=""</div>
<div>+OPT_VERBOSE=""</div>
<div> </div>
<div>-while getopts "c:hlnqRsy0" opt $*; do</div>
<div>+while getopts "Ac:hlnqRsvy0" opt $*; do</div>
<div> case $opt in</div>
<div>+<span style="white-space:pre-wrap"> </span>A) OPT_AUTOSTRIPE=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>c) OPT_STRIPE_COUNT=$OPTARG;;</div>
<div> <span style="white-space:pre-wrap"> </span>l) OPT_NLINK=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>n) OPT_DRYRUN=n; OPT_YES=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>q) ECHO=:;;</div>
<div> <span style="white-space:pre-wrap"> </span>R) OPT_RESTRIPE=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>s) OPT_CHECK="";;</div>
<div>+<span style="white-space:pre-wrap"> </span>v) OPT_VERBOSE=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>y) OPT_YES=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>0) OPT_NULL=y;;</div>
<div> <span style="white-space:pre-wrap"> </span>h|\?) usage;;</div>
<div>@@ -69,6 +75,16 @@</div>
<div> <span style="white-space:pre-wrap"> </span>echo "$(basename $0) error: The -c <stripe_count> option may not" 1>&2</div>
<div> <span style="white-space:pre-wrap"> </span>echo "be specified at the same time as the -R option." 1>&2</div>
<div> <span style="white-space:pre-wrap"> </span>exit 1</div>
<div>+elif [ "$OPT_STRIPE_COUNT" -a "$OPT_AUTOSTRIPE" ]; then</div>
<div>+<span style="white-space:pre-wrap"> </span>echo ""</div>
<div>+<span style="white-space:pre-wrap"> </span>echo "$(basename $0) error: The -c <stripe_count> option may not" 1>&2</div>
<div>+<span style="white-space:pre-wrap"> </span>echo "be specified at the same time as the -A option." 1>&2</div>
<div>+<span style="white-space:pre-wrap"> </span>exit 1</div>
<div>+elif [ "$OPT_AUTOSTRIPE" -a "$OPT_RESTRIPE" ]; then</div>
<div>+<span style="white-space:pre-wrap"> </span>echo ""</div>
<div>+<span style="white-space:pre-wrap"> </span>echo "$(basename $0) error: The -A option may not be specified at" 1>&2</div>
<div>+<span style="white-space:pre-wrap"> </span>echo "the same time as the -R option." 1>&2</div>
<div>+<span style="white-space:pre-wrap"> </span>exit 1</div>
<div> fi</div>
<div> </div>
<div> if [ -z "$OPT_YES" ]; then</div>
<div>@@ -107,7 +123,7 @@</div>
<div> <span style="white-space:pre-wrap"> </span>$ECHO -n "$OLDNAME: "</div>
<div> </div>
<div> <span style="white-space:pre-wrap"> </span># avoid duplicate stat if possible</div>
<div>-<span style="white-space:pre-wrap"> </span>TYPE_LINK=($(LANG=C stat -c "%h %F" "$OLDNAME" || true))</div>
<div>+<span style="white-space:pre-wrap"> </span>TYPE_LINK=($(LANG=C stat -c "%h %F %s" "$OLDNAME" || true))</div>
<div> </div>
<div> <span style="white-space:pre-wrap"> </span># skip non-regular files, since they don't have any objects</div>
<div> <span style="white-space:pre-wrap"> </span># and there is no point in trying to migrate them.</div>
<div>@@ -127,11 +143,6 @@</div>
<div> <span style="white-space:pre-wrap"> </span>continue</div>
<div> <span style="white-space:pre-wrap"> </span>fi</div>
<div> </div>
<div>-<span style="white-space:pre-wrap"> </span>if [ "$OPT_DRYRUN" ]; then</div>
<div>-<span style="white-space:pre-wrap"> </span>echo -e "dry run, skipped"</div>
<div>-<span style="white-space:pre-wrap"> </span>continue</div>
<div>-<span style="white-space:pre-wrap"> </span>fi</div>
<div>-</div>
<div> <span style="white-space:pre-wrap"> </span>if [ "$OPT_RESTRIPE" ]; then</div>
<div> <span style="white-space:pre-wrap"> </span>UNLINK=""</div>
<div> <span style="white-space:pre-wrap"> </span>else</div>
<div>@@ -140,16 +151,43 @@</div>
<div> <span style="white-space:pre-wrap"> </span># then we don't need to do this getstripe/mktemp stuff.</div>
<div> <span style="white-space:pre-wrap"> </span>UNLINK="-u"</div>
<div> </div>
<div>-<span style="white-space:pre-wrap"> </span>[ "$OPT_STRIPE_COUNT" ] && COUNT=$OPT_STRIPE_COUNT ||</div>
<div>-<span style="white-space:pre-wrap"> </span>COUNT=$($LFS getstripe -c "$OLDNAME" \</div>
<div>-<span style="white-space:pre-wrap"> </span>2> /dev/null)</div>
<div> <span style="white-space:pre-wrap"> </span>SIZE=$($LFS getstripe $LFS_SIZE_OPT "$OLDNAME" \</div>
<div> <span style="white-space:pre-wrap"> </span> 2> /dev/null)</div>
<div>+<span style="white-space:pre-wrap"> </span>if [ "$OPT_AUTOSTRIPE" ]; then</div>
<div>+<span style="white-space:pre-wrap"> </span>FILE_SIZE=${TYPE_LINK[3]}<br>
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<div>+<span style="white-space:pre-wrap"> </span># (math in bash is dumb, so depend on common tools, and there are options for that...)</div>
<div>+<span style="white-space:pre-wrap"> </span># Stripe Count = Log2(size_in_GB)</div>
<div>+<span style="white-space:pre-wrap"> </span>#COUNT=$(echo $FILE_SIZE | awk '{printf "%.0f\n",log($1/1024/1024/1024)/log(2)}')</div>
<div>+<span style="white-space:pre-wrap"> </span>#COUNT=$(printf "%.0f\n" $(echo "l($FILE_SIZE/1024/1024/1024) / l(2)" | bc -l))</div>
<div>+<span style="white-space:pre-wrap"> </span>COUNT=$(echo "l($FILE_SIZE/1024/1024/1024) / l(2) + 1" | bc -l | cut -d . -f 1)</div>
<div>+<span style="white-space:pre-wrap"> </span># Stripe Count = size_in_GB</div>
<div>+<span style="white-space:pre-wrap"> </span>#COUNT=$(echo "scale=0; $FILE_SIZE/1024/1024/1024" | bc -l | cut -d . -f 1)</div>
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<div>Instead of involving "bc", which is not guaranteed to be installed, why not just have a simple "divide by 2, increment stripe_count" loop after converting bytes to GiB? That would be a few cycles for huge files, but probably still faster than fork/exec
of an external binary as it could be at most 63 - 30 = 33 loops and usually many fewer.</div>
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<div>Cheers, Andreas</div>
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<div>+<span style="white-space:pre-wrap"> </span>[ "$COUNT" -lt 1 ] && COUNT=1</div>
<div>+<span style="white-space:pre-wrap"> </span># (does it make sense to skip the file if old</div>
<div>+<span style="white-space:pre-wrap"> </span># and new stripe count are identical?)</div>
<div>+<span style="white-space:pre-wrap"> </span>else</div>
<div>+<span style="white-space:pre-wrap"> </span>[ "$OPT_STRIPE_COUNT" ] && COUNT=$OPT_STRIPE_COUNT ||</div>
<div>+<span style="white-space:pre-wrap"> </span>COUNT=$($LFS getstripe -c "$OLDNAME" \</div>
<div>+<span style="white-space:pre-wrap"> </span>2> /dev/null)</div>
<div>+<span style="white-space:pre-wrap"> </span>fi</div>
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<div> <span style="white-space:pre-wrap"> </span>[ -z "$COUNT" -o -z "$SIZE" ] && UNLINK=""</div>
<div>-<span style="white-space:pre-wrap"> </span>SIZE=${LFS_SIZE_OPT}${SIZE}</div>
<div> <span style="white-space:pre-wrap"> </span>fi</div>
<div> </div>
<div>+<span style="white-space:pre-wrap"> </span>if [ "$OPT_DRYRUN" ]; then</div>
<div>+<span style="white-space:pre-wrap"> </span>if [ "$OPT_VERBOSE" ]; then</div>
<div>+<span style="white-space:pre-wrap"> </span>echo -e "dry run, would use count=${COUNT} size=${SIZE}"</div>
<div>+<span style="white-space:pre-wrap"> </span>else</div>
<div>+<span style="white-space:pre-wrap"> </span>echo -e "dry run, skipped"</div>
<div>+<span style="white-space:pre-wrap"> </span>fi</div>
<div>+<span style="white-space:pre-wrap"> </span>continue</div>
<div>+<span style="white-space:pre-wrap"> </span>fi</div>
<div>+<span style="white-space:pre-wrap"> </span>if [ "$OPT_VERBOSE" ]; then</div>
<div>+<span style="white-space:pre-wrap"> </span>echo -n "(count=${COUNT} size=${SIZE}) "</div>
<div>+<span style="white-space:pre-wrap"> </span>fi</div>
<div>+</div>
<div>+<span style="white-space:pre-wrap"> </span>[ "$SIZE" ] && SIZE=${LFS_SIZE_OPT}${SIZE}</div>
<div>+</div>
<div> <span style="white-space:pre-wrap"> </span># first try to migrate inside lustre</div>
<div> <span style="white-space:pre-wrap"> </span># if failed go back to old rsync mode</div>
<div> <span style="white-space:pre-wrap"> </span>if [[ $RSYNC_MODE == false ]]; then</div>
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