3 Sep
2014
3 Sep
'14
5:34 a.m.
Hi Luvers,
is it possible for a file to be written to and the time stamp not changed?
Specifically, is it possible for a swap file to be changed and the time
stamp not changed? OR if the time stamp on a swap file has not changed
for several days, does that mean that the system has not used swap for
that time, since this is the only swap space available? or could it be
that the kernel process that initiates swap does not update the
filesystem fully?
Thanks
Daniel.
3 Sep
3 Sep
5:47 a.m.
Hi Daniel,
On Wed, September 3, 2014 4:34 pm, Daniel J Jitnah wrote:
is it possible for a file to be written to and the
time stamp not changed?
You use the touch command to revert back to the time before the change
cf.
http://askubuntu.com/questions/62492/how-can-i-change-the-date-modified-cre…
Not exactly elegant, but it could work in a pinch.
--
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Mngmnt) (Chifley)
mobile: 0432 255 208
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5:59 a.m.
On 03/09/14 15:34, Daniel J Jitnah wrote:
Hi Luvers,
is it possible for a file to be written to and the time stamp not changed?
Specifically, is it possible for a swap file to be changed and the time
stamp not changed? OR if the time stamp on a swap file has not changed
for several days, does that mean that the system has not used swap for
that time, since this is the only swap space available? or could it be
that the kernel process that initiates swap does not update the
filesystem fully?
I'm just surmising here so don't take it as gospel.
If you have a pre-existing file, as swap files usually are, then you can
use dd to write into the guts of it without altering any of the time stamps.
Certainly this is the case if you do the writing at the device level
without the filesystem unmounted, but that is just plain tedious.
It should be easy for you to experiment.
7:04 a.m.
On Wed, 3 Sep 2014 15:59:48 Allan Duncan wrote:
On 03/09/14 15:34, Daniel J Jitnah wrote:
> Hi Luvers,
>
> is it possible for a file to be written to and the time stamp not changed?
>
> Specifically, is it possible for a swap file to be changed and the time
> stamp not changed? OR if the time stamp on a swap file has not changed
> for several days, does that mean that the system has not used swap for
> that time, since this is the only swap space available? or could it be
> that the kernel process that initiates swap does not update the
> filesystem fully?
# free
total used free shared buffers cached
Mem: 12498384 11585772 912612 0 52412 1308840
-/+ buffers/cache: 10224520 2273864
Swap: 2097148 7260 2089888
# ls -l /root/swap
-rw-r--r--. 1 root root 2.0G Oct 18 2012 /root/swap
# uptime
07:01:40 up 66 days, 22:41, 1 user, load average: 0.61, 0.49, 0.59
It appears that using the swap file doesn't operate in the same manner as other
filesystem access. I guess that the filesystem provides a list of blocks on
disk that are then accessed without the filesystem. The above is from one of
my systems, timestamp says that it was last written in 2012 but 7M of swap was
written in the last 66 days.
I'm just surmising here so don't take it as
gospel.
If you have a pre-existing file, as swap files usually are, then you can
use dd to write into the guts of it without altering any of the time stamps.
No, dd uses regular write mechanisms and the time stamp will be updated.
However you could write to it and then set the time stamp back.
Certainly this is the case if you do the writing at
the device level
without the filesystem unmounted, but that is just plain tedious.
swap is different.
--
My Main Blog http://etbe.coker.com.au/
My Documents Blog http://doc.coker.com.au/
8:08 a.m.
On 03/09/14 17:04, Russell Coker wrote:
On Wed, 3 Sep 2014 15:59:48 Allan Duncan wrote:
Thats most
useful! Thanks.
It makes sense! When you think about it, the time stamp on a swap file
is hardly useful information. Besides updating the time stamp each time
swap is written to could be regarded as wasted critical cpu cycles.
On 03/09/14 15:34, Daniel J Jitnah wrote:
> Hi Luvers,
>
> is it possible for a file to be written to and the time stamp not changed?
>
> Specifically, is it possible for a swap file to be changed and the time
> stamp not changed? OR if the time stamp on a swap file has not changed
> for several days, does that mean that the system has not used swap for
> that time, since this is the only swap space available? or could it be
> that the kernel process that initiates swap does not update the
> filesystem fully?
# free
total used free shared buffers cached
Mem: 12498384 11585772 912612 0 52412 1308840
-/+ buffers/cache: 10224520 2273864
Swap: 2097148 7260 2089888
# ls -l /root/swap
-rw-r--r--. 1 root root 2.0G Oct 18 2012 /root/swap
# uptime
07:01:40 up 66 days, 22:41, 1 user, load average: 0.61, 0.49, 0.59 It appears that using the swap file doesn't
operate in the same manner as other
filesystem access.
That does raise more questions on having swap on a ssd only
system.
There are VPS providers now that are offering VPS's using ssd's. It
looks that they are not also providing swap. For small VPS say 1gb ram
or less, swap is desirable.
Daniel.
I guess that the filesystem provides a list of blocks
on
disk that are then accessed without the filesystem. The above is from one of
my systems, timestamp says that it was last written in 2012 but 7M of swap was
written in the last 66 days.
I'm just surmising here so don't take it
as gospel.
If you have a pre-existing file, as swap files usually are, then you can
use dd to write into the guts of it without altering any of the time stamps.
No,
dd uses regular write mechanisms and the time stamp will be updated.
However you could write to it and then set the time stamp back.
Certainly this is the case if you do the writing
at the device level
without the filesystem unmounted, but that is just plain tedious.
swap is
different.
1:36 a.m.
On 04/09/14 11:19, Trent W. Buck wrote:
Daniel J Jitnah
<djitnah(a)greenwareit.com.au> writes:
Thanks for that. Will definitely
investigate further!
D.
That does raise more questions on having swap on
a ssd only system.
As a rule, I don't recommend swap.
For small VPS say 1gb ram
or less, swap is desirable.
Why not zram?
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5 Sep
5 Sep
12:30 a.m.
Daniel J Jitnah wrote:
I drop this bodge into rc.local.
It turns a quarter of your RAM into compressed swap (using DEFLATE).
Using LZ4 would probably be better if you have a new kernel.
modprobe zram &&
awk '/^MemTotal/{printf("%dk",$2*0.25)}' /proc/meminfo
Why not zram?
Thanks for that. Will definitely investigate further! /sys/block/zram0/disksize &&
mkswap
/dev/zram0 &&
swapon /dev/zram0 &&
sysctl vm.swappiness=80
This is how it looks in action:
total used free
Mem: 1884 954 930
-/+ buffers/cache: 222 1662
Swap: 511 0 511
So you see, I have no experience about whether it's any GOOD :-)
IIRC the swappiness=80 was to encourage that last zero to grow.
4 Sep
4 Sep
9:48 a.m.
On Wed, 3 Sep 2014 18:08:36 Daniel J Jitnah wrote:
Not providing swap by default doesn't stop you from setting it up in most
cases. It's possible for a VPS provider to force you to use a kernel compiled
without swap support but I'm not aware of examples of anyone doing that.
I've been using linode.com for a long time, they have always supported swap
partitions through the management GUI. Recently they transitioned to using
SSD for everything.
--
My Main Blog http://etbe.coker.com.au/
My Documents Blog http://doc.coker.com.au/
--
Sent from my Samsung Galaxy Note 2 with K-9 Mail.
It makes sense! When you think about it, the time
stamp on a swap file
is hardly useful information.
The fact that you wanted to see it in the first place indicates that it is
useful on occasion.
Besides updating the time stamp each time
swap is written to could be regarded as wasted critical cpu cycles.
CPU is not an issue. Disk access is an issue. For small writes if the mtime
is updated you can double the disk IO requirement and worse than double it in
the case of filesystems with journalled metadata.
It appears
that using the swap file doesn't operate in the same manner as
other filesystem access.
That does raise more questions on having swap on a ssd only system.
There are VPS providers now that are offering VPS's using ssd's. It
looks that they are not also providing swap. For small VPS say 1gb ram
or less, swap is desirable. 
3 Sep
3 Sep
6:22 a.m.
On 3 September 2014 15:34, Daniel J Jitnah <djitnah(a)greenwareit.com.au> wrote:
Specifically, is it possible for a swap file to be changed and the time
stamp not changed? OR if the time stamp on a swap file has not changed
for several days, does that mean that the system has not used swap for
that time, since this is the only swap space available? or could it be
that the kernel process that initiates swap does not update the
filesystem fully?
I think [1] pswpin in /proc/vmstat reports swap usage, if that helps you.
[1] http://www.linuxinsight.com/proc_vmstat.html
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9 comments
6 participants
participants (6)
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Allan Duncan -
Daniel J Jitnah -
David -
Lev Lafayette -
Russell Coker -
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