I think it has no mean in RAID1 mode. It is used in RAID0,4,5,6,10 modes. You can see in man mdadm.
Toni Mas 2018-03-07 23:06 GMT+01:00 Darac Marjal <mailingl...@darac.org.uk>: > > > On 07/03/18 21:13, Steve Keller wrote: > > I have a RAID1 array with 2 disks (/dev/sda1 and /dev/sdb1) of 2 TB > > each. By running mdadm -X /dev/sda1 I see that the chunk size is 64 MB: > > > > # mdadm -X /dev/sda1 > > Filename : /dev/sda1 > > Magic : 6d746962 > > Version : 4 > > UUID : 300551ed:f6690dfb:1c939898:af5509c6 > > Events : 2599997 > > Events Cleared : 2599997 > > State : OK > > Chunksize : 64 MB > > Daemon : 5s flush period > > Write Mode : Normal > > Sync Size : 1953381376 (1862.89 GiB 2000.26 GB) > > Bitmap : 29807 bits (chunks), 2 dirty (0.0%) > > > > What exactly does the chunk sized mean? My question is how reads and > > writes on an array are done. Will the kernel always read or write a > > complete chunk? If so, does that mean that writing a single 4 KB > > block to a file system will cause a 64 MB read, i.e one chunk, change > > the 4 KB block in that chunk and write back the 64 MB chunk? > > Yes, my understanding is that chunk size is the size of area upon which > parity is calculated, or the size of data which is allocated before > moving onto the next drive etc. > > My guess, though, is that there is a balance to be struck. Yes, if the > chunk size is small, then there is very little write amplification. But > if the chunk size is too small, then you need to wait for that chunk to > pass the read-write head again, you need to be switching between sectors > very often etc. With a bigger chunk, you can take better advantage of > caching. These days, 64Mb is a relatively small amount to pull into a > buffer, it can be pulled in, modified and rewritten virtually > instantanously. > > There's a nice article on the effect of different chunk sizes here: > http://louwrentius.com/linux-raid-level-and-chunk-size-the-benchmarks.html > > > > > Wouldn't that mean a massive performance problem? > > > > Steve > > > > >
