Re: Why O_SYNC and fsync are slow in Linux?
On Wed, 16 May 2001, Heikki Tuuri wrote: > On Red Hat 6.2 and 7.? Intel big block writes are very slow if > I open the file with O_SYNC. I call pwrite to write 1 MB chunks to > the file, and I get only 1 MB/s write speed. If I open without O_SYNC > and call fsync only after writing the whole 100 MB file, > I get 5 MB/s. I got the same adequate speed 5 MB/s with 16 MB writes > after which I called fdatasync. When you open with O_SYNC, the data must go all the way to disk on every write call. This means you get at least one disk access for every write, and possibly more if the writes are large (>64k). When you don't use O_SYNC and only flush after all writes have been submitted by the application, then the kernel is able to combine writes in the cache and at the blk dev layer. Therefore you end up with fewer accesses to the physical disk, which makes it much faster. > On a Linux-Compaq Alpha I measured the following: if I open with O_SYNC, > I can flush the end of my file (it is a log file) to > disk 170 times / second. If I do not open with O_SYNC, > but call fsync or fdatasync after each write, I get only 50 > writes/second. This is generally the case. If you need to sync every write, O_SYNC is usually faster than fsync. If you don't need to sync every individual write, then a single fsync after the last write is the fastest to get all the data to disk. > > On the Red Hat 7.? I get 500 writes per second if I open with O_SYNC. > That is too much because the disk does not rotate > 500 rotations/second. Does the disk fool the operating > system to believe a write has ended while it has not? Are you using an IDE disk? If so, it probably has a huge cache. -Lance - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why O_SYNC and fsync are slow in Linux?
On Wed, 16 May 2001, Heikki Tuuri wrote: On Red Hat 6.2 and 7.? Intel big block writes are very slow if I open the file with O_SYNC. I call pwrite to write 1 MB chunks to the file, and I get only 1 MB/s write speed. If I open without O_SYNC and call fsync only after writing the whole 100 MB file, I get 5 MB/s. I got the same adequate speed 5 MB/s with 16 MB writes after which I called fdatasync. When you open with O_SYNC, the data must go all the way to disk on every write call. This means you get at least one disk access for every write, and possibly more if the writes are large (64k). When you don't use O_SYNC and only flush after all writes have been submitted by the application, then the kernel is able to combine writes in the cache and at the blk dev layer. Therefore you end up with fewer accesses to the physical disk, which makes it much faster. On a Linux-Compaq Alpha I measured the following: if I open with O_SYNC, I can flush the end of my file (it is a log file) to disk 170 times / second. If I do not open with O_SYNC, but call fsync or fdatasync after each write, I get only 50 writes/second. This is generally the case. If you need to sync every write, O_SYNC is usually faster than fsync. If you don't need to sync every individual write, then a single fsync after the last write is the fastest to get all the data to disk. On the Red Hat 7.? I get 500 writes per second if I open with O_SYNC. That is too much because the disk does not rotate 500 rotations/second. Does the disk fool the operating system to believe a write has ended while it has not? Are you using an IDE disk? If so, it probably has a huge cache. -Lance - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Why O_SYNC and fsync are slow in Linux?
(Please CC your replies to me because I am not on the list.) Hi! Does anyone happen to know who is responsible for the file cache and disk management in Linux? On different systems I have measured strange differences in performance depending on whether I open a file with O_SYNC and let the operating system do the flushing of the file to disk after each write, or if I open without O_SYNC, and call fsync myself. Some observations: On Red Hat 6.2 and 7.? Intel big block writes are very slow if I open the file with O_SYNC. I call pwrite to write 1 MB chunks to the file, and I get only 1 MB/s write speed. If I open without O_SYNC and call fsync only after writing the whole 100 MB file, I get 5 MB/s. I got the same adequate speed 5 MB/s with 16 MB writes after which I called fdatasync. On a Linux-Compaq Alpha I measured the following: if I open with O_SYNC, I can flush the end of my file (it is a log file) to disk 170 times / second. If I do not open with O_SYNC, but call fsync or fdatasync after each write, I get only 50 writes/second. On the Red Hat 7.? I get 500 writes per second if I open with O_SYNC. That is too much because the disk does not rotate 500 rotations/second. Does the disk fool the operating system to believe a write has ended while it has not? On Windows NT I have not noticed such performance problems if I use non-buffered i/o to a file. I have written a database engine InnoDB under MySQL and bumped into these problems on Linux. Regards, Heikki Tuuri Innobase Oy http://www.innobase.fi - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Why O_SYNC and fsync are slow in Linux?
(Please CC your replies to me because I am not on the list.) Hi! Does anyone happen to know who is responsible for the file cache and disk management in Linux? On different systems I have measured strange differences in performance depending on whether I open a file with O_SYNC and let the operating system do the flushing of the file to disk after each write, or if I open without O_SYNC, and call fsync myself. Some observations: On Red Hat 6.2 and 7.? Intel big block writes are very slow if I open the file with O_SYNC. I call pwrite to write 1 MB chunks to the file, and I get only 1 MB/s write speed. If I open without O_SYNC and call fsync only after writing the whole 100 MB file, I get 5 MB/s. I got the same adequate speed 5 MB/s with 16 MB writes after which I called fdatasync. On a Linux-Compaq Alpha I measured the following: if I open with O_SYNC, I can flush the end of my file (it is a log file) to disk 170 times / second. If I do not open with O_SYNC, but call fsync or fdatasync after each write, I get only 50 writes/second. On the Red Hat 7.? I get 500 writes per second if I open with O_SYNC. That is too much because the disk does not rotate 500 rotations/second. Does the disk fool the operating system to believe a write has ended while it has not? On Windows NT I have not noticed such performance problems if I use non-buffered i/o to a file. I have written a database engine InnoDB under MySQL and bumped into these problems on Linux. Regards, Heikki Tuuri Innobase Oy http://www.innobase.fi - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
