Yes. The drive treats some portion of cells as SLC, which having only two charge states is a lot faster and is used as cache. As with any cache-enabled drive, if that cache fills up either due to misaligned flush cycles or simply data coming in faster than it can flush, you’ll see a performance cliff.
Also, uniquely for reasons I don’t yet understand, as the drive fills up, the size of that cache area decreases linearly until it hits a certain minimum. Thus an empty drive will have more cache in service than a 70% full drive. This article includes 8x10 color glossy pictures with circles and arrows and a paragraph on the back describing what each one is about. https://www.howtogeek.com/428869/ssds-are-getting-denser-and-slower-thanks-to-qlc-flash/ Which raises some interesting questions about the proper use of TRIM both by RBD clients and by ceph-osd, especially when dmcrypt is used. — aad > Are you saying that the write performance becomes bad (90MB/sec) for long > lasting *continuous* writing? (after filling up a write buffer or such) > > But given time to empty that buffer again, it should again write with the > normal higher speed? > > So in applications with enough variation between reading and writing, they > could still perform good enough? > > MJ > > On 3/6/20 2:06 PM, vita...@yourcmc.ru wrote: >> Hi, >> Current QLC drives are total shit in terms of steady-state performance. >> First 10-100 GB of data is written into the SLC cache which is fast, but >> then the drive switches to its QLC memory and even the linear write >> performance drops to ~90 MB/s which is actually worse than with HDDs! >> So, try to run a long linear write test and check the performance after >> writing a lot of data. >>> Last monday I performed a quick test with those two disks already, >>> probably not that relevant, but posting it anyway: > _______________________________________________ > ceph-users mailing list -- ceph-users@ceph.io > To unsubscribe send an email to ceph-users-le...@ceph.io _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
