5 ms·

No, the write load is nearly constant. With a fixed-size buffer in the drive, you're pretty close to optimal use case for SMR. There's no source of variance e

No, the write load is nearly constant. With a fixed-size buffer in the drive, you're pretty close to optimal use case for SMR. There's no source of variance except for where the SMR mechanism adds it, and that's analogous to a garbage collector sweep. Size and frequency depend on dirty rate.

throwaway3563 7y ago
The write load will be nearly constant but it won’t be sequential. The host device is going to have to update file system data structures periodically as it is writing the video stream; those structures will be at various different places on the drive. So the write load will look like 99% sequential, 1% random. That small amount of random writes in a non-contiguous stream is enough to cause SMR-related rewrite stuttering, especially since with DMSMR the host has no idea how logical blocks presented by the drive map to SMR regions.
chiph 7y ago
Even on a freshly formatted drive, you'd want your write rate to stay below what the SMR rewrite process can handle. If the drive fills up and it needs to delete something to make room before writing new video, its ability to keep up with the incoming video data is going to be reduced. Lots of recording systems won't record unless the camera sees motion. Or they record at a much lower frame rate until motion is detected & then go into high speed mode to capture fine details. So that helps, and the CMR buffer also helps. But if the recording fills up the CMR buffer, you're back to incoming data potentially arriving faster than SMR can rewrite. Time to drop back to 1080 HD cameras from 4k cameras maybe. Or reduce the number of cameras. edit This is comparing a SMR to a similar CMR drive. Obviously, any drive can run into trouble if you exceed its write capabilities. But a SMR drive's write rate is going to be lower than a CMR drive, once it's buffer area is full. And especially if you're doing random writes.

throwaway3563 7y ago

The write load will be nearly constant but it won’t be sequential. The host device is going to have to update file system data structures periodically as it is writing the video stream; those structures will be at various different places on the drive. So the write load will look like 99% sequential, 1% random. That small amount of random writes in a non-contiguous stream is enough to cause SMR-related rewrite stuttering, especially since with DMSMR the host has no idea how logical blocks presented by the drive map to SMR regions.

chiph 7y ago

Even on a freshly formatted drive, you'd want your write rate to stay below what the SMR rewrite process can handle. If the drive fills up and it needs to delete something to make room before writing new video, its ability to keep up with the incoming video data is going to be reduced. Lots of recording systems won't record unless the camera sees motion. Or they record at a much lower frame rate until motion is detected & then go into high speed mode to capture fine details. So that helps, and the CMR buffer also helps. But if the recording fills up the CMR buffer, you're back to incoming data potentially arriving faster than SMR can rewrite. Time to drop back to 1080 HD cameras from 4k cameras maybe. Or reduce the number of cameras. edit This is comparing a SMR to a similar CMR drive. Obviously, any drive can run into trouble if you exceed its write capabilities. But a SMR drive's write rate is going to be lower than a CMR drive, once it's buffer area is full. And especially if you're doing random writes.