In my last article
Linux SSD partition alignment – problems with external USB-to-SATA controllers – I
I wrote about different partition alignments parted and YaST’s Partitioner (Opensuse) applied when one and the same SSD (a Samsung 850 Pro) was attached
- to external USB-to-SATA controllers and the USB bus of a Linux system (1 MiB-alignment: start sectors as multiples of 2048 logical 512 byte blocks)
- or directly to an internal SATA-III-controller of a Linux system (alignment to multiples of 65535 logical 512 byte blocks).
We saw that different controllers led to the detection of different disk topology parameters (I/O limits) by the Linux system via the libblkid library. For one and the same SSD different values were reported by different controllers e.g. for the following parameters (I/O Limits data) :
physical_block_size, minimum_io_size, optimal_io_size
We saw in addition that even different Linux disk tools may report different values; e.g. fdisk showed a different value [512 byte] for the “optimal_io_size” for the SSD on the SATA bus than e.g. lsblk and parted [0].
Guided by a Red Hat article https://people.redhat.com/msnitzer/docs/io-limits.txt we came to the conclusion that at least parted and YaST’s Partitioner use heuristic rules for its alignment decisions. The rules take into account the values for disk “I/O Limits” parameters. They are consistent with a default of “optimal” for the alignment parameter of parted and provide a decision when “the value for “optimal_io_size” is found to be zero. By applying these rules we could explain why we got different partition offsets and alignments for one and the same disk when it was attached to different controllers.
But this insight left us in an uncomfortable situation:
- Should we cling to the chosen settings when we use the SSD on external controllers, only? Can we partition SSDs on an external USB-to-SATA controller and move them later directly to a SATA-bus without adjusting partition borders? We saw that “parted” would complain about misalignment when SSD partitions were prepared on a different controller.
- As many people discuss the importance of partition alignment for SSD performance – will we see a noticeable drop in performance when we read/write to “misaligned” partitions?
- We saw that at least a JMicron controller indicated a bundling of 8 logical 512 byte blocks into a 4096 byte (fake?) “physical block”. Another question might therefore be what happens after an installation and something is written by Grub to the first sector of a disk with GPT layout – and maybe assuming some wrong disk topology? This is not so far fetched as one may think; see the third link at the bottom for a disaster with an MBR.
I cannot answer all these questions in general. But in this article I will at least look a bit into performance issues and answer the last question for my test situation.