> -----Original Message----- > From: Austin S Hemmelgarn [mailto:ahferroin7@xxxxxxxxx] > Sent: Tuesday, September 8, 2015 7:56 AM > Subject: Re: mkfs.btrfs cannot find rotational file for SSD detection for > a pmem device > > On 2015-09-06 13:51, Elliott, Robert (Persistent Memory) wrote: ... > > The impact looks limited to the print and causing it to not > > automatically disable "metadata duplication on a single device." > This is an issue inherent in the current pmem driver however, it should > be fixed there and not in mkfs.btrfs, as other filesystems make > decisions based on this file also, as does the I/O scheduler, and some > block storage servers. > ... The rotational file does exist, at: /sys/devices/LNXSYSTM\:00/LNXSYBUS\:00/ACPI0012\:00/ndbus1/region0/namespace0.0/block/pmem0/queue/rotational One or more functions are having trouble parsing that 108-byte string ... mkfs.btrfs's is_ssd, libblkid's blkid_devno_to_wholedisk, or libblkid's sysfs_devno_to_wholedisk. I'm not sure where the breakdown occurs. This is reminiscent of an issue that numactl has parsing the path to get to .../device/numa_node (rather than .../queue/rotational). It was confused by not finding "/devices/pci" in a path for a storage device. > This gets tricky though because pmem isn't > technically a block device at the low level, and doesn't use some parts > of the block layer that most other block devices do. > > On that note however, if the pmem device is backed by actual RAM and not > flash storage (and most of them are from what I've seen), then the only > advantage of using single metadata mode over dup is space savings, as > RAM is not (usually) write limited. pmem devices will be a mix ranging from flash-backed DRAM to new technologies like 3D Crosspoint, usually offering high performance and good wearout characteristics. The btrfs driver does detect it as SSD after mkfs.btrfs did not: kernel: BTRFS info (device pmem0): disk space caching is enabled kernel: BTRFS: has skinny extents kernel: BTRFS: flagging fs with big metadata feature kernel: BTRFS: detected SSD devices, enabling SSD mode -- To unsubscribe from this list: send the line "unsubscribe util-linux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
