Bcache writeback_percent max value
I'm tryng to set
writeback_percent
at a value > 40 but it only accept value between 0 and 40.
If i set
echo 50 > /sys/block/bcache0/bcache/writeback_percent
then when i read the value
more /sys/block/bcache0/bcache/writeback_percent
i have 40.
For value<=40 the settings work fine.
My setting for cache type are
more /sys/block/bcache0/bcache/cache_mode
writethrough [writeback] writearound none
I know this is dangerous but this is not a problem for me.
As far as i understand writeback_percent is the % used from dirty data in cache, why i can't use 90% or 100% of available space?
May be i dont' understand quite well this settings?
bcache
asked Sep 10 '17 at 13:06
Antimo
82
add a comment |
I'm tryng to set
writeback_percent
at a value > 40 but it only accept value between 0 and 40.
If i set
echo 50 > /sys/block/bcache0/bcache/writeback_percent
then when i read the value
more /sys/block/bcache0/bcache/writeback_percent
i have 40.
For value<=40 the settings work fine.
My setting for cache type are
more /sys/block/bcache0/bcache/cache_mode
writethrough [writeback] writearound none
I know this is dangerous but this is not a problem for me.
As far as i understand writeback_percent is the % used from dirty data in cache, why i can't use 90% or 100% of available space?
May be i dont' understand quite well this settings?
bcache
asked Sep 10 '17 at 13:06
Antimo
82
Thank you @sourcedj, but may be i'm wrong but previously i was able to use a value > 40. This are file systems in use only for automatic test purpose so data are copied from a source that is not cached , backupped and so on so no problem if i lost this data. Anyway i understand that probability of data loss increase but i can't find any info about this limit, so i ask here about some clarification. And anyway my English is not perfect :)
– Antimo
Sep 10 '17 at 19:36
Sorry, I was criticizing only a small detail. Don't worry about it. It sounds like I wasn't very helpful! I don't know why bcache is not accepting a value > 40. It's a perfectly reasonable question to have, hopefully someone who knows more will see your post.
– sourcejedi
Sep 10 '17 at 19:44
add a comment |
I'm tryng to set
writeback_percent
at a value > 40 but it only accept value between 0 and 40.
If i set
echo 50 > /sys/block/bcache0/bcache/writeback_percent
then when i read the value
more /sys/block/bcache0/bcache/writeback_percent
i have 40.
For value<=40 the settings work fine.
My setting for cache type are
more /sys/block/bcache0/bcache/cache_mode
writethrough [writeback] writearound none
I know this is dangerous but this is not a problem for me.
As far as i understand writeback_percent is the % used from dirty data in cache, why i can't use 90% or 100% of available space?
May be i dont' understand quite well this settings?
bcache
asked Sep 10 '17 at 13:06
Antimo
82
I'm tryng to set
writeback_percent
at a value > 40 but it only accept value between 0 and 40.
If i set
echo 50 > /sys/block/bcache0/bcache/writeback_percent
then when i read the value
more /sys/block/bcache0/bcache/writeback_percent
i have 40.
For value<=40 the settings work fine.
My setting for cache type are
more /sys/block/bcache0/bcache/cache_mode
writethrough [writeback] writearound none
I know this is dangerous but this is not a problem for me.
As far as i understand writeback_percent is the % used from dirty data in cache, why i can't use 90% or 100% of available space?
May be i dont' understand quite well this settings?
bcache
bcache
asked Sep 10 '17 at 13:06
Antimo
82
asked Sep 10 '17 at 13:06
Antimo
82
asked Sep 10 '17 at 13:06
Antimo
82
asked Sep 10 '17 at 13:06
Antimo
82
asked Sep 10 '17 at 13:06
Antimo
82
82
Thank you @sourcedj, but may be i'm wrong but previously i was able to use a value > 40. This are file systems in use only for automatic test purpose so data are copied from a source that is not cached , backupped and so on so no problem if i lost this data. Anyway i understand that probability of data loss increase but i can't find any info about this limit, so i ask here about some clarification. And anyway my English is not perfect :)
– Antimo
Sep 10 '17 at 19:36
Sorry, I was criticizing only a small detail. Don't worry about it. It sounds like I wasn't very helpful! I don't know why bcache is not accepting a value > 40. It's a perfectly reasonable question to have, hopefully someone who knows more will see your post.
– sourcejedi
Sep 10 '17 at 19:44
add a comment |
Thank you @sourcedj, but may be i'm wrong but previously i was able to use a value > 40. This are file systems in use only for automatic test purpose so data are copied from a source that is not cached , backupped and so on so no problem if i lost this data. Anyway i understand that probability of data loss increase but i can't find any info about this limit, so i ask here about some clarification. And anyway my English is not perfect :)
– Antimo
Sep 10 '17 at 19:36
Sorry, I was criticizing only a small detail. Don't worry about it. It sounds like I wasn't very helpful! I don't know why bcache is not accepting a value > 40. It's a perfectly reasonable question to have, hopefully someone who knows more will see your post.
– sourcejedi
Sep 10 '17 at 19:44
Thank you @sourcedj, but may be i'm wrong but previously i was able to use a value > 40. This are file systems in use only for automatic test purpose so data are copied from a source that is not cached , backupped and so on so no problem if i lost this data. Anyway i understand that probability of data loss increase but i can't find any info about this limit, so i ask here about some clarification. And anyway my English is not perfect :)
– Antimo
Sep 10 '17 at 19:36
Thank you @sourcedj, but may be i'm wrong but previously i was able to use a value > 40. This are file systems in use only for automatic test purpose so data are copied from a source that is not cached , backupped and so on so no problem if i lost this data. Anyway i understand that probability of data loss increase but i can't find any info about this limit, so i ask here about some clarification. And anyway my English is not perfect :)
– Antimo
Sep 10 '17 at 19:36
Sorry, I was criticizing only a small detail. Don't worry about it. It sounds like I wasn't very helpful! I don't know why bcache is not accepting a value > 40. It's a perfectly reasonable question to have, hopefully someone who knows more will see your post.
– sourcejedi
Sep 10 '17 at 19:44
Sorry, I was criticizing only a small detail. Don't worry about it. It sounds like I wasn't very helpful! I don't know why bcache is not accepting a value > 40. It's a perfectly reasonable question to have, hopefully someone who knows more will see your post.
– sourcejedi
Sep 10 '17 at 19:44
add a comment |
2 Answers
2
active
oldest
votes
This is hardcoded value in the bcache drive code - linux/drivers/md/bcache/writeback.h. The only way to change this limit is to rebuild the driver from source.
answered Oct 19 at 20:38
V.Gorbunov
261
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
add a comment |
According to
https://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt
writeback_percent
If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate.
So this is a target objective for the amount of dirty cache. In that sense I would rather set it to 0%. And a value of 100% means "wait until the cache is full to start writing it to the backing device".
Since the dirty data is potentially at risk if you lose the cache SSD, why would you set such high amounts? I guess this is the reason why the developer hardcoded an upper boundary
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
answered Oct 27 at 9:11
hjohanns
1
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
This is hardcoded value in the bcache drive code - linux/drivers/md/bcache/writeback.h. The only way to change this limit is to rebuild the driver from source.
answered Oct 19 at 20:38
V.Gorbunov
261
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
add a comment |
This is hardcoded value in the bcache drive code - linux/drivers/md/bcache/writeback.h. The only way to change this limit is to rebuild the driver from source.
answered Oct 19 at 20:38
V.Gorbunov
261
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
add a comment |
This is hardcoded value in the bcache drive code - linux/drivers/md/bcache/writeback.h. The only way to change this limit is to rebuild the driver from source.
answered Oct 19 at 20:38
V.Gorbunov
261
This is hardcoded value in the bcache drive code - linux/drivers/md/bcache/writeback.h. The only way to change this limit is to rebuild the driver from source.
answered Oct 19 at 20:38
V.Gorbunov
261
answered Oct 19 at 20:38
V.Gorbunov
261
answered Oct 19 at 20:38
V.Gorbunov
261
answered Oct 19 at 20:38
V.Gorbunov
261
261
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
add a comment |
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
True, except wrong source file, at least in v4.19. It's in sysfs.c: elixir.bootlin.com/linux/v4.19.9/source/drivers/md/bcache/… . I don't know the relation between the two values.
– sourcejedi
Dec 16 at 13:53
add a comment |
According to
https://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt
writeback_percent
If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate.
So this is a target objective for the amount of dirty cache. In that sense I would rather set it to 0%. And a value of 100% means "wait until the cache is full to start writing it to the backing device".
Since the dirty data is potentially at risk if you lose the cache SSD, why would you set such high amounts? I guess this is the reason why the developer hardcoded an upper boundary
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
answered Oct 27 at 9:11
hjohanns
1
add a comment |
According to
https://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt
writeback_percent
If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate.
So this is a target objective for the amount of dirty cache. In that sense I would rather set it to 0%. And a value of 100% means "wait until the cache is full to start writing it to the backing device".
Since the dirty data is potentially at risk if you lose the cache SSD, why would you set such high amounts? I guess this is the reason why the developer hardcoded an upper boundary
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
answered Oct 27 at 9:11
hjohanns
1
add a comment |
According to
https://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt
writeback_percent
If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate.
So this is a target objective for the amount of dirty cache. In that sense I would rather set it to 0%. And a value of 100% means "wait until the cache is full to start writing it to the backing device".
Since the dirty data is potentially at risk if you lose the cache SSD, why would you set such high amounts? I guess this is the reason why the developer hardcoded an upper boundary
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
answered Oct 27 at 9:11
hjohanns
1
According to
https://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt
writeback_percent
If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate.
So this is a target objective for the amount of dirty cache. In that sense I would rather set it to 0%. And a value of 100% means "wait until the cache is full to start writing it to the backing device".
Since the dirty data is potentially at risk if you lose the cache SSD, why would you set such high amounts? I guess this is the reason why the developer hardcoded an upper boundary
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
answered Oct 27 at 9:11
hjohanns
1
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
edited Dec 16 at 11:56
Rui F Ribeiro
38.9k1479129
38.9k1479129
answered Oct 27 at 9:11
hjohanns
1
answered Oct 27 at 9:11
hjohanns
1
answered Oct 27 at 9:11
hjohanns
1
1
add a comment |
add a comment |
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Thank you @sourcedj, but may be i'm wrong but previously i was able to use a value > 40. This are file systems in use only for automatic test purpose so data are copied from a source that is not cached , backupped and so on so no problem if i lost this data. Anyway i understand that probability of data loss increase but i can't find any info about this limit, so i ask here about some clarification. And anyway my English is not perfect :)
– Antimo
Sep 10 '17 at 19:36
Sorry, I was criticizing only a small detail. Don't worry about it. It sounds like I wasn't very helpful! I don't know why bcache is not accepting a value > 40. It's a perfectly reasonable question to have, hopefully someone who knows more will see your post.
– sourcejedi
Sep 10 '17 at 19:44