Slow recognition of free space on deleted filesystems
I have noticed this happening twice in the last few days:
I will cd into a filesystem (/ora1 or /data_c01) and run an rm -rf * &
A few minutes later my job will show as done and the jobs command shows no active background jobs, but when I do a df -h I will still see the filesystem as having data in it. If I cd to the directory and run an ls. I see no data, even with a ls -a.
About 5 minutes later a df -h will show the filesystem as being empty? Why the delay?
The delay is bacause df reports the actual amount of space on the disk, but the delete requests are still in the hard disk's buffers.
To maximise performance, the hard-disk buffers only get committed when the disk is on a low load, when they get full, after a certain amount of time, or when they are explicitly synchronised.
You can see the amount of disk space by passing df the --sync option, which will request an immediate commit of the disk buffers and wait for that request to finish.
You can also mount a device with the sync and dirsync options (set in /etc/fstab before mounting), in order to have the device be accessed synchronously (i.e. buffers committed after every read or write). This is slow.
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