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<p class="MsoNormal">I am looking for more details on the “-n size=65536” option in mkfs.xfs. The question is the memory allocation this option generates. The system is Redhat EL 7.0 (3.10.0-229.1.2.el7.x86_64).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We have been getting this memory allocation deadlock message in the /var/log/messages file. The file system is used for ceph OSD and it has about 531894 files.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Oct 6 07:11:09 abc-ceph1-xyz kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x8250)<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Reading this post, <a href="https://access.redhat.com/solutions/532663">
https://access.redhat.com/solutions/532663</a> it discussed the “memory allocation deadlock” message. It looks like the root cause is a very fragmented file. Is setting the “-n size=65536” option on the file system causes (directly or indirectly) the “memory
allocation deadlock” error?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Here is a note on the “-n size=65536”. The FAQ note is more on the CPU and IO performance.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><a href="http://xfs.org/index.php/XFS_FAQ">http://xfs.org/index.php/XFS_FAQ</a><o:p></o:p></p>
<p class="MsoNormal">“<o:p></o:p></p>
<p class="MsoNormal">Q: Performance: mkfs.xfs -n size=64k option<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Asking the implications of that mkfs option on the XFS mailing list,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Dave Chinner explained it this way:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Inodes are not stored in the directory structure, only the directory entry name<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">and the inode number. Hence the amount of space used by a directory entry is<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">determined by the length of the name.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">There is extra overhead to allocate large directory blocks (16 pages instead of<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">one, to begin with, then there's the vmap overhead, etc), so for small<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">directories smaller block sizes are faster for create and unlink operations.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">For empty directories, operations on 4k block sized directories consume roughly<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">50% less CPU that 64k block size directories. The 4k block size directories<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">consume less CPU out to roughly 1.5 million entries where the two are roughly<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">equal. At directory sizes of 10 million entries, 64k directory block operations<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">are consuming about 15% of the CPU that 4k directory block operations consume.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">In terms of lookups, the 64k block directory will take less IO but consume more<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">CPU for a given lookup. Hence it depends on your IO latency and whether<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">directory readahead can hide that latency as to which will be faster. e.g. For<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">SSDs, CPU usage might be the limiting factor, not the IO. Right now I don't<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">have any numbers on what the difference might be - I'm getting 1 billion inode<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">population issues worked out first before I start on measuring cold cache<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">lookup times on 1 billion files....</span><o:p></o:p></p>
<p class="MsoNormal">“<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Al Lau<o:p></o:p></p>
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