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On Tue, Jun 23, 2020 at 12:14 PM Sid Spry <sid@××××.us> wrote: |
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> |
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> So if I'm understanding properly most drive firmware won't let you |
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> operate the device in an append-only mode? |
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So, there are several types of SMR drives. |
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There are host-managed, drive-managed, and then hybrid devices that |
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default to drive-managed for compatibility reasons but the host can |
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send them a command to take full control so that it is the same as |
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host-managed. |
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A host-managed drive just does what the host tells it to. If the host |
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tells it to do a write that obliterates some other data on the disk, |
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the drive just does it, and it is the job of the host |
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OS/filesystem/application to make sure that they protect any data they |
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care about. At the drive level these perform identically to CMR |
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because they just seek and write like any other drive. At the |
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application level these could perform differently since the |
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application might end up having to work around the drive. However, |
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these drives are generally chosen for applications where this is not a |
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big problem or where the problems can be efficiently mitigated. |
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A drive-managed drive just looks like a regular drive to the host, and |
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it ends up having to do a lot of read-before-rewrite operations |
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because the host is treating it like it is CMR but the drive has to |
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guarantee that nothing gets lost. A drive-managed disk has no way to |
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operate in "append-only" mode. I'm not an expert in ATA but I believe |
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disks are just given an LBA and a set of data to write. Without |
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support for TRIM the drive has no way to know if it is safe to |
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overwrite nearby cylinders, which means it has to preserve them. |
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The biggest problem is that the vendors were trying to conceal the |
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nature of the drives. If they advertised TRIM support it would be |
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pretty obvious they were SMR. |
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> If any do I suspect |
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> NILFS (https://en.wikipedia.org/wiki/NILFS) may be a good choice: |
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> |
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> "NILFS is a log-structured filesystem, in that the storage medium is treated |
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> like a circular buffer and new blocks are always written to the end. [...]" |
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> |
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On a host-managed disk this would perform the same as on a CMR disk, |
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with the possible exception of any fixed metadata (I haven't read the |
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gory details on the filesystem). If it has no fixed metadata (without |
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surrounding unused tracks) then it would have no issues at all on SMR. |
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F2FS takes a similar approach for SSDs, though it didn't really take |
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off because ext4's support is good enough and I suspect that modern |
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SSDs are fast enough at erasing. |
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As I mentioned SSDs have the same issue as SMR, just the scale of the |
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problem is much smaller. The erase blocks are much smaller so less |
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data needs to be read/rewritten following an erase, and then the |
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operations are all just vastly faster. Taking 3x longer on a 1us |
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operation is far different from taking 100x longer on a 100ms |
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operation. |
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-- |
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Rich |
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