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Can full cache prevent disc writes (Withdrawn)
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nlsa8z6zoz7lyih3ap
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Location: Canada

PostPosted: Wed Dec 23, 2015 5:09 pm    Post subject: Can full cache prevent disc writes (Withdrawn) Reply with quote


Further tests show that while I can sometimes repeat this problem, I can't consistently replicate it on the basis of the conditions described below. For that reason I am withdrawing the thread. Without the ability to replicate, there really is nothing to discuss.)




The Situation:
On a laptop I try to copy the root filesystem to a backup with

Code:
find /|afio -vp   -Y"/root-backup/*" -Y"exclusions" ...''-Y"exclusions"  /root-backup


On my Desktop this always works. On my wife's laptop it fails to copy some directories.

Question: On the laptop is it possible that the memory cache fills up and that some cached but pending writes to the disc are dropped?

PS Lots of disc space is available to complete the above.
PPS If I use afio to make an afio archive file of / and then use afio to install the archive to /root-backup, then everything get copied over.

Any ideas or insights would be appreciated.


Last edited by nlsa8z6zoz7lyih3ap on Thu Dec 24, 2015 5:13 am; edited 1 time in total
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nlsa8z6zoz7lyih3ap
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Joined: 25 Sep 2007
Posts: 376
Location: Canada

PostPosted: Wed Dec 23, 2015 6:46 pm    Post subject: Reply with quote

I think that the problem is with afio rather than with cache and buffer causing disc write to be dropped as when I
run (as root) every minute
Code:
 sync &&  echo 3 > /proc/sys/vm/drop_caches


The problem remains. Any insights would be appreciated.
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nlsa8z6zoz7lyih3ap
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Joined: 25 Sep 2007
Posts: 376
Location: Canada

PostPosted: Wed Dec 23, 2015 10:52 pm    Post subject: Reply with quote

I have sorted out how to make the afio -vp work and how to make it fail, although I don't have a full understanding of what has happened and don't know if the failure
is a bug or an expected and reasonable result.

The problem seems to lie in how find interacts with locales.
(1) Failure when the default locale is en_CA.utf8
(2) Success when I add
Code:
export LC_ALL="C"

to the start of the backup script in which find /|afio -vp .................................... is invoked.

In case (1), find / doesn't output the directory that was missed in the backup. (/etc was one of them)

So it appears that find does not work correctly when the locale is en_CA.utf8 rather than C.

I still do not understand this and hence feel distinctly uncomfortable about a subtle failure that I did not see coming, especially as it involves a backup script.

I would appreciate comments or explanation from someone who does understand this.
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