+ LWN.net + News from the source +
++In the real world, however, it turns out to be better, sometimes, to spread +things out. Imagine setting up a system with users' home directories in +/home. If all the first-level directories within /home +(i.e. the home directories for numerous users) are placed next to each +other, there may be no space left for the contents of those directores. +User files thus end up being placed far from the directories that contain +them, and performance suffers. The ext2 filesystem has suffered from this +sort of performance degradation for some time. +
+The 2.5.46 kernel contains a new block allocator which attempts to address +this problem. The new scheme, borrowed from BSD, is named the "Orlov +allocator," after its creator Grigory Orlov; he has posted a brief +description of the technique as it is used in the BSD kernels. The +Linux implementation, as implemented by +Alexander Viro, Andrew Morton, and Ted Ts'o, uses a similar technique but +adds a few changes. +
+Essentially, the Orlov algorithm tries to spread out "top-level" +directories, on the assumption that they are unrelated to each other. +Directories created in the root directory of a filesystem are considered +top-level directories; Ted has added a special inode flag that allows +the system administrator to mark other directories as being top-level +directories as well. If /home lives in the root filesystem (and +people do set up systems that way), a simple chattr command will +make the system treat it as a top-level directory. +
+When creating a directory which is not in a top-level directory, the +Orlov algorithm tries, as before, to put it into the same cylinder group as +its parent. A little more care is taken, however, to ensure that the +directory's contents will also be able to fit into that cylinder group; if +there are not many inodes or blocks available in the group, the directory +will be placed in a different cylinder group which has more resources +available. The result of all this, hopefully, is much better locality for +files which are truly related to each other and likely to be accessed +together. +
+As of this writing, only one benchmark
+result with the new allocator has been posted. The results are
+promising: the time required to traverse through a Linux kernel tree (a
+dauntingly big thing, these days) was reduced by 30% or so. The Orlov
+scheme needs more rigorous benchmarking; it also needs some serious stress
+testing to demonstrate that performance does not degrade as the filesystem
+is changed over time. But the initial results are encouraging. Linux has, once
+again, benefitted from the ability to borrow good ideas from other free
+kernels.
+ +
Posted Nov 7, 2002 14:29 UTC (Thu) by jlnance (guest, #4081) + [Link] +
+ I think you have this backwards. The old ext2/3 code always tried to+ +
+
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