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How do filesystem commit interval options interact with vm.dirty_expire_centisecs?

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1

Question

How do filesystem "commit interval" options interact with vm.dirty_expire_centisecs? What happens when one is shorter than the other? Does it ever make sense to set these differently?

My understanding is that filesystem commit interval settings control how often the filesystem will proactively write dirty data and metadata to disk, even when fsync hasn't been called by the application.

Separately, vm.dirty_expire_centisecs seems to have a similar role, but at the VM layer rather than the filesystem layer.

---

References

ext4 commit mount option:

Ext4 can be told to sync all its data and metadata every 'nrsec' seconds. The default value is 5 seconds. This means that if you lose your power, you will lose as much as the latest 5 seconds of work (your filesystem will not be damaged though, thanks to the journaling).

btrfs commit mount option:

Set the interval of periodic commit. Higher values defer data being synced to permanent storage with obvious consequences when the system crashes.

Note, I'm leaving out XFS for now, as its fs.xfs.xfssyncd_centisecs option appears to apply to metadata only.

vm.dirty_expire_centisecs:

This tunable is used to define when dirty data is old enough to be eligible
for writeout by the kernel flusher threads. It is expressed in 100'ths
of a second. Data which has been dirty in-memory for longer than this
interval will be written out next time a flusher thread wakes up.

linux filesystems

share|improve this question

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

add a comment | 

1

Question

How do filesystem "commit interval" options interact with vm.dirty_expire_centisecs? What happens when one is shorter than the other? Does it ever make sense to set these differently?

My understanding is that filesystem commit interval settings control how often the filesystem will proactively write dirty data and metadata to disk, even when fsync hasn't been called by the application.

Separately, vm.dirty_expire_centisecs seems to have a similar role, but at the VM layer rather than the filesystem layer.

---

References

ext4 commit mount option:

Ext4 can be told to sync all its data and metadata every 'nrsec' seconds. The default value is 5 seconds. This means that if you lose your power, you will lose as much as the latest 5 seconds of work (your filesystem will not be damaged though, thanks to the journaling).

btrfs commit mount option:

Set the interval of periodic commit. Higher values defer data being synced to permanent storage with obvious consequences when the system crashes.

Note, I'm leaving out XFS for now, as its fs.xfs.xfssyncd_centisecs option appears to apply to metadata only.

vm.dirty_expire_centisecs:

This tunable is used to define when dirty data is old enough to be eligible
for writeout by the kernel flusher threads. It is expressed in 100'ths
of a second. Data which has been dirty in-memory for longer than this
interval will be written out next time a flusher thread wakes up.

linux filesystems

share|improve this question

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

add a comment | 

Question

How do filesystem "commit interval" options interact with vm.dirty_expire_centisecs? What happens when one is shorter than the other? Does it ever make sense to set these differently?

My understanding is that filesystem commit interval settings control how often the filesystem will proactively write dirty data and metadata to disk, even when fsync hasn't been called by the application.

Separately, vm.dirty_expire_centisecs seems to have a similar role, but at the VM layer rather than the filesystem layer.

---

References

ext4 commit mount option:

Ext4 can be told to sync all its data and metadata every 'nrsec' seconds. The default value is 5 seconds. This means that if you lose your power, you will lose as much as the latest 5 seconds of work (your filesystem will not be damaged though, thanks to the journaling).

btrfs commit mount option:

Set the interval of periodic commit. Higher values defer data being synced to permanent storage with obvious consequences when the system crashes.

Note, I'm leaving out XFS for now, as its fs.xfs.xfssyncd_centisecs option appears to apply to metadata only.

vm.dirty_expire_centisecs:

This tunable is used to define when dirty data is old enough to be eligible
for writeout by the kernel flusher threads. It is expressed in 100'ths
of a second. Data which has been dirty in-memory for longer than this
interval will be written out next time a flusher thread wakes up.

linux filesystems

share|improve this question

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

share|improve this question

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

share|improve this question

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

asked Dec 19 '18 at 8:41

pauldoopauldoo

15428

1 Answer
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0

It just influence the frequency actual logic is with kernel. I don't think if you give less value it will be effective, it take kernel defaults.

 .procname = "dirty_expire_centisecs",
 .data = &dirty_expire_interval,
 .maxlen = sizeof(dirty_expire_interval),
 .mode = 0644,
 .proc_handler = proc_dointvec_minmax,
 .extra1 = &zero,

When the first page is dirtied in an inode, the current time is recorded in the inode. When this time gets older than dirty_expire_centisecs, all dirty pages in the inode are written. So with this mechanism in mind the behavior you describe looksexpected to me.

unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */

share|improve this answer

answered May 4 at 11:12

asktyagiasktyagi

1176

add a comment | 

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0

It just influence the frequency actual logic is with kernel. I don't think if you give less value it will be effective, it take kernel defaults.

 .procname = "dirty_expire_centisecs",
 .data = &dirty_expire_interval,
 .maxlen = sizeof(dirty_expire_interval),
 .mode = 0644,
 .proc_handler = proc_dointvec_minmax,
 .extra1 = &zero,

When the first page is dirtied in an inode, the current time is recorded in the inode. When this time gets older than dirty_expire_centisecs, all dirty pages in the inode are written. So with this mechanism in mind the behavior you describe looksexpected to me.

unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */

share|improve this answer

answered May 4 at 11:12

asktyagiasktyagi

1176

add a comment | 

0

It just influence the frequency actual logic is with kernel. I don't think if you give less value it will be effective, it take kernel defaults.

 .procname = "dirty_expire_centisecs",
 .data = &dirty_expire_interval,
 .maxlen = sizeof(dirty_expire_interval),
 .mode = 0644,
 .proc_handler = proc_dointvec_minmax,
 .extra1 = &zero,

When the first page is dirtied in an inode, the current time is recorded in the inode. When this time gets older than dirty_expire_centisecs, all dirty pages in the inode are written. So with this mechanism in mind the behavior you describe looksexpected to me.

unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */

share|improve this answer

answered May 4 at 11:12

asktyagiasktyagi

1176

add a comment | 

It just influence the frequency actual logic is with kernel. I don't think if you give less value it will be effective, it take kernel defaults.

 .procname = "dirty_expire_centisecs",
 .data = &dirty_expire_interval,
 .maxlen = sizeof(dirty_expire_interval),
 .mode = 0644,
 .proc_handler = proc_dointvec_minmax,
 .extra1 = &zero,

When the first page is dirtied in an inode, the current time is recorded in the inode. When this time gets older than dirty_expire_centisecs, all dirty pages in the inode are written. So with this mechanism in mind the behavior you describe looksexpected to me.

unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */

share|improve this answer

answered May 4 at 11:12

asktyagiasktyagi

1176

 .procname = "dirty_expire_centisecs",
 .data = &dirty_expire_interval,
 .maxlen = sizeof(dirty_expire_interval),
 .mode = 0644,
 .proc_handler = proc_dointvec_minmax,
 .extra1 = &zero,

unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */

share|improve this answer

answered May 4 at 11:12

asktyagiasktyagi

1176

answered May 4 at 11:12

asktyagiasktyagi

1176

answered May 4 at 11:12

asktyagiasktyagi

1176