> On Wed, Oct 6, 2021 at 3:37 PM Laurence Perkins <lperk...@openeye.net> wrote: > > I think what to look for there would be if there's a way to align the BTRFS > > chunks to the SMR blocks. > > There are definitely ways to implement filesystems that are more compatible > with SMR. You basically want something like a log-based filesystem. A COW > filesystem is actually a really good candidate as they don't do in-place > writes ever, and all you need to do is defer block frees and garbage collect > and so on to make it more log-based.
COW is a good compromise. Log-based filesystems like NILFS2 generally have terrible performance. Their only advantage in this situation is that they don't suffer any detectable performance loss from SMR. EXT4 has new formatting options that tell it to sequentialize the metadata writes as much as possible. Supposedly it helps quite a bit, but I haven't tested it myself yet. > > One of the issues though is that these drives obfuscate how they work. > The filesystem has no way to intentionally write to the CMR vs SMR regions. It depends on the drive. There are ATA commands you can use and at least some drives will tell you what their zones are and can be told to give the host more control over what goes where. https://zonedstorage.io/getting-started/smr-disk/ has a good description. Definitely avoid the Drive Managed Interface ones except maybe for cheap, bulk storage. But the Host Aware and Host Managed ones can perform pretty well on some IO loads with proper FS tuning. > > This is why drive-managed SMR really shouldn't be a thing. > Host-managed SMR makes a LOT more sense, because then the filesystem can > mitigate most of the issues and not end up fighting the drive firmware, whose > behavior isn't even standardized. Yup. Only reason for drive-managed is for drop-in replacement of low-throughput systems that couldn't otherwise handle it. > > > But the manufacturers decided to continue manufacturing CMR disks for the > > surveillance industry, so I haven't had to worry about it just yet. > > There are lots of CMR drives out there. It is just that you have to be > careful as nothing is well-documented and it is all subject to change. It is > like trying to figure out how many channels a DIMM has or what its timing > capabilities are. > > I saw a good price on an Exos drive and I believe those are all CMR. > This is why I use slickdeals - you can set up searches and get alerts when a > price drops. It also picks up stuff like Best Buy who often has some of the > best prices on USB enclosures for whatever reason when they go on sale. Almost everything marketed as being for the surveillance industry is going to be CMR. We often run the drives at 80% or better throughput and a lot of it is non-sequential. SMRs just can't keep up. So there was a lot of pushback from the surveillance industry about not changing the underlying storage technology without clearly labeling it as such. Prior to that the announcement from Seagate and I think WD was that everything larger than 12TB was going to be SMR. (I work on the software side of things, I get hardware stuff second hand, so my understanding may be fuzzy.) But currently the WD Purples and the Seagate Skyhawks should all be CMR. Obviously I have no power to hold them to that, so do not take this as any kind of guarantee. But if they change it without warning they're likely to have a lot of unhappy customers. > > -- > Rich > > LMP
