[SOLVED] Files with apparent size smaller than actual size?

pigsa · 04-14-2015, 09:46 PM

I note some files with smaller apparent size that actual size, i.e., opposite to sparse file. It's a difference of 100MB, not a few KB. What sort of file is this, why, and how would one create such file?

Code:

[pigsa@pigsa journal]# du -h  *
101M    WiredTigerLog.0000000001
101M    WiredTigerPreplog.0000000001
[pigsa@pigsa journal]# du -h --apparent-size *
144K    WiredTigerLog.0000000001
128     WiredTigerPreplog.0000000001

Note:
1) Ext4 filesystem
2) These are oplog files of mongodb 3 wiredtiger storage engine... But I am interested in the phenenom of small apparent size, rather than what wiredtiger is.

veerain · 04-14-2015, 11:13 PM

I think they are indeed sparse files.

astrogeek · 04-14-2015, 11:22 PM

Welcome to LQ!

It is the opposite of what you think it to be.

From man du:

Code:

--apparent-size
       print  apparent  sizes, rather than disk usage; although the 
       apparent size is usually smaller, it may be larger due 
       to holes in ('sparse') files, internal fragmentation, indirect 
       blocks, and  the like

pigsa · 04-15-2015, 01:04 AM

Eventually found that it is possible to produce such phenomenon using the fallocate command