What is the different between the Cache Device and Journal Device?
jingxi02
4 Posts
April 27, 2020, 12:30 amQuote from jingxi02 on April 27, 2020, 12:30 amWhen choosing a disk at the setup screen, it gives the 3 options. OSD is where the data going save to. The Journal is a read/write cache for the OSD. What about the Cache device? I can't find the info in the manual.
When choosing a disk at the setup screen, it gives the 3 options. OSD is where the data going save to. The Journal is a read/write cache for the OSD. What about the Cache device? I can't find the info in the manual.
admin
2,930 Posts
April 27, 2020, 4:00 pmQuote from admin on April 27, 2020, 4:00 pmthe journal contains:
wal : write ahead log, speeds up small writes by using a 2 phase commit
db : stores metadata such as object location, putting on ssds speeds up read/writes
this provided by Ceph.
Cache is different, as data is stored on the fast device for a long time, we use dm-writecache which is provided by the kernel.
the journal contains:
wal : write ahead log, speeds up small writes by using a 2 phase commit
db : stores metadata such as object location, putting on ssds speeds up read/writes
this provided by Ceph.
Cache is different, as data is stored on the fast device for a long time, we use dm-writecache which is provided by the kernel.
brent
11 Posts
April 30, 2020, 10:01 pmQuote from brent on April 30, 2020, 10:01 pmWhat are the recommendations for when to use caching and/or journaling with PetaSAN latest version? I noticed when I set a disk as a journal it created 64GB partitions for as many OSD's as it could, but when I set a disk as a cache device it only linked to three OSD's and the rest didn't get created. Does a caching device only work for 3 disks?
What are the recommendations for when to use caching and/or journaling with PetaSAN latest version? I noticed when I set a disk as a journal it created 64GB partitions for as many OSD's as it could, but when I set a disk as a cache device it only linked to three OSD's and the rest didn't get created. Does a caching device only work for 3 disks?
admin
2,930 Posts
May 1, 2020, 12:56 pmQuote from admin on May 1, 2020, 12:56 pmWith hdds, you should use an SSD journal, it improves both reading and writing.
If your workload has a lot of small writes, write cache makes a big difference in this case.
The way they are setup is a bit different, journal is a Ceph device, it allocates a fixed amount / partition size each time you add an OSD until you have no more space. Write cache is something we added support in PetaSAN, you need to specify the number of partitions upfront when creating a cache device, we allow 1 to 8, although 2 to 4 is more ideal.
With hdds, you should use an SSD journal, it improves both reading and writing.
If your workload has a lot of small writes, write cache makes a big difference in this case.
The way they are setup is a bit different, journal is a Ceph device, it allocates a fixed amount / partition size each time you add an OSD until you have no more space. Write cache is something we added support in PetaSAN, you need to specify the number of partitions upfront when creating a cache device, we allow 1 to 8, although 2 to 4 is more ideal.
What is the different between the Cache Device and Journal Device?
jingxi02
4 Posts
Quote from jingxi02 on April 27, 2020, 12:30 amWhen choosing a disk at the setup screen, it gives the 3 options. OSD is where the data going save to. The Journal is a read/write cache for the OSD. What about the Cache device? I can't find the info in the manual.
When choosing a disk at the setup screen, it gives the 3 options. OSD is where the data going save to. The Journal is a read/write cache for the OSD. What about the Cache device? I can't find the info in the manual.
admin
2,930 Posts
Quote from admin on April 27, 2020, 4:00 pmthe journal contains:
wal : write ahead log, speeds up small writes by using a 2 phase commit
db : stores metadata such as object location, putting on ssds speeds up read/writes
this provided by Ceph.
Cache is different, as data is stored on the fast device for a long time, we use dm-writecache which is provided by the kernel.
the journal contains:
wal : write ahead log, speeds up small writes by using a 2 phase commit
db : stores metadata such as object location, putting on ssds speeds up read/writes
this provided by Ceph.
Cache is different, as data is stored on the fast device for a long time, we use dm-writecache which is provided by the kernel.
brent
11 Posts
Quote from brent on April 30, 2020, 10:01 pmWhat are the recommendations for when to use caching and/or journaling with PetaSAN latest version? I noticed when I set a disk as a journal it created 64GB partitions for as many OSD's as it could, but when I set a disk as a cache device it only linked to three OSD's and the rest didn't get created. Does a caching device only work for 3 disks?
What are the recommendations for when to use caching and/or journaling with PetaSAN latest version? I noticed when I set a disk as a journal it created 64GB partitions for as many OSD's as it could, but when I set a disk as a cache device it only linked to three OSD's and the rest didn't get created. Does a caching device only work for 3 disks?
admin
2,930 Posts
Quote from admin on May 1, 2020, 12:56 pmWith hdds, you should use an SSD journal, it improves both reading and writing.
If your workload has a lot of small writes, write cache makes a big difference in this case.
The way they are setup is a bit different, journal is a Ceph device, it allocates a fixed amount / partition size each time you add an OSD until you have no more space. Write cache is something we added support in PetaSAN, you need to specify the number of partitions upfront when creating a cache device, we allow 1 to 8, although 2 to 4 is more ideal.
With hdds, you should use an SSD journal, it improves both reading and writing.
If your workload has a lot of small writes, write cache makes a big difference in this case.
The way they are setup is a bit different, journal is a Ceph device, it allocates a fixed amount / partition size each time you add an OSD until you have no more space. Write cache is something we added support in PetaSAN, you need to specify the number of partitions upfront when creating a cache device, we allow 1 to 8, although 2 to 4 is more ideal.