jack@cwi.nl (Jack Jansen) (01/12/89)
Does anyone happen to have statistics around of the number of blocks that are written to disk that belong to files that have been removed already? This came up as a side-issue in a conversation today, and I was just wondering how much I/O bandwidth unix is spending on flushing deleted temporary files to disk. All facts, pointers and hunches welcome... -- -- Fight war, not wars | Jack Jansen, jack@cwi.nl Destroy power, not people! -- Crass | (or mcvax!jack)
edler@cmcl2.NYU.EDU (Jan Edler) (01/13/89)
In article <7819@boring.cwi.nl> jack@cwi.nl (Jack Jansen) writes: >Does anyone happen to have statistics around of the number of >blocks that are written to disk that belong to files that have >been removed already? This topic came up once before, but I don't have a copy of my response at that time. This should be similar. We optimize these away on our Ultracomputer prototypes. We use the V7 filesystem with ordered writes and a 2KB blocksize. We've configured 128 buffers into the system, and on a recent 3-day period, collected the following data: number of buffer cache lookups: 1996814 number of hits: 1722425 (86% hit rate) number of actual fs disk reads: 222451 number of actual fs disk writes: 123421 number of disk blocks freed: 56164 number of those still in cache: 23188 (41%) number of those marked delayed write: 6639 (12% overall) I don't know how typical our workload is, and this is just one data point, but there it is. 12% of our disk blocks freed might have been needlessly written to disk without our optimization, adding 1% to our overall filesystem disk I/O rate. Of course, some of them might have been re-allocated and re-referenced again before being written to disk anyway, but we do a sync every 45 seconds. You decide how worthwhile it all is. Jan Edler NYU Ultracomputer Project edler@nyu.edu (212) 998-3353