On Saturday 16 November 2024 20:13:30 GMT Rich Freeman wrote: > On Sat, Nov 16, 2024 at 2:47 PM Michael <confabul...@kintzios.com> wrote:
> > What differs with > > HM-SMR instructions is the host is meant to take over the management of > > random writes and submit these as sequential whole band streams to the > > drive to be committed without a read-modify-write penalty. I suppose for > > the host to have to read the whole band first from the drive, modify it > > and then submit it to the drive to write it as a whole band will be > > faster than letting the drive manage this operation internally and > > getting its internal cache full. > > I doubt this would be any faster with a host-managed drive. The same > pattern of writes is going to incur the same penalties. > > The idea of a host-managed drive is to avoid the random writes in the > first place, and the need to do the random reads. For this to work > the host has to know where the boundaries of the various regions are > and where it is safe to begin writes in each region. The random reads do not incur a time penalty, it is the R-M-W ops that cost time. The host don't need to know where bands start and finish, only needs to submit data in whole sequential streams, so they can be written directly to the disk as in a CMR. As long as data and metadata are submitted and written directly, the SMR would be alike a CMR in terms of its performance. > Sure, a host could just use software to make the host-managed drive > behave the same as a drive-managed drive, but there isn't much benefit > there. You'd want to use a log-based storage system/etc to just avoid > the random writes entirely. You might not even want to use a POSIX > filesystem on it. > > > This > > will not absolve the drive firmware from having to manage its own trim > > operations and the impact metadata changes could have on the drive, but > > some timing optimisation is perhaps reasonable. > > Why would a host-managed SMR drive have ANY trim operations? What > does trimming even mean on a host-managed drive? > > Trimming is the act of telling the drive that it is safe to delete a > block without preserving it. A host-managed drive shouldn't need to > be concerned with preserving any data during a write operation. If it > is told to write something, it will just overwrite the data in the > subsequent overlapping cylinders. I assumed, may be wrongly, there is still an STL function performed by the controller on HM-SMRs, to de-allocate deleted data bands whenever files are deleted, perform secure data deletions via its firmware, etc. However, I can see if this is managed at the fs journal layer the drive controller could be dumb in this respect. Perhaps what I had read referred to HM-SMR 'aware' drives, which may behave as DM-SMRs depending on the OS capability. It would be interesting to see how different fs types perform on DM-SMRs. Looking at used drives on ebay they are rather still rather pricey for me to splash out on one of them, but since my PVR 14 year old WD SATA 2 refuses to die they may get cheaper by the time I need a replacement.
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