Messages in this thread | | | From | Jim Meyering <> | Subject | efficient access to "rotational"; new fcntl? | Date | Fri, 18 Sep 2009 21:31:50 +0200 |
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About a year ago, I fixed GNU rm to avoid quadratic readdir/stat seek penalties on ext3 and ext4 that became overwhelming on directories with many entries (1M entries started to look like infloop or DoS).
From coreutils-7.0 NEWS:
chgrp, chmod, chown, chcon, du, rm: now all display linear performance, even when operating on million-entry directories on ext3 and ext4 file systems. Before, they would exhibit O(N^2) performance, due to linear per-entry seek time cost when operating on entries in readdir order. Rm was improved directly, while the others inherit the improvement from the newer version of fts in gnulib.
To do that efficiently, I changed the core readdir loop (mainly via gnulib's fts.c) to preprocess entries, when needed, by sorting them on inode, and *then* processing them. This optimization is enabled only when the affected file system is of a type likely to benefit, and when the number of directory entries is large enough to matter.
However, with e.g., an ext4 partition on non-rotational hardware like an SSD, that preprocessing is unnecessary and in fact wasted effort. I'd like to avoid the waste by querying the equivalent of /sys/.../rotational, via a syscall like fcntl or statvfs, given a file descriptor.
Is there an efficient way to get that single bit?
IMHO, locating and opening-then-reading a file like /sys/devices/pci0000:00/0000:00:1f.2/host4/target4:0:0/4:0:0:0/block/sdc/queue/rotational is not appropriate in the context of a low-level library function like fts.
Jim
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