Why buffered writes are sometimes stalled

Many people think buffered write (write()/pwrite()) is fast because it does not do disk access. But this is not always true. Buffered write sometimes does disk access by itself, or waits for some disk accesses by other threads. Here are three common cases where write() takes longer time (== causing stalls). 1. Read Modify Write Suppose the following logic. Opening aaa.dat without O_DIRECT/O_SYNC,

[efcl]

"stable page writes" 書き込みサイズを4Kbとかの単位に切るのはWriteのみにしてスループットを上げる

[ono_matope]

書き込みをOSのページサイズにalignするのって、そうすることでページキャッシュのRead-Modify-Writeを単純なWriteにできるからだったのか。目からウロコだ。 / あとで Stable Page Write 調べる。

[hiboma]

RMW

[ogijun]

あとで読む

[yass]

" 1. write() does disk read when needed. To avoid this issue you need to append a file, not overwrite. Or use OS page aligned writes.   2. write() may be blocked for "stable page writes". To avoid this issue you need to use newer Linux kernel supporting disabling stable page writes. "

[hirose31]

write

[sh2]

Oracle DatabaseではREDOログファイルのブロックサイズを選べるようになっていて http://docs.oracle.com/cd/E49329_01/server.121/b71301/onlineredo.htm#CHDFBDJC 、O_DSYNCを付けています