I need to pass a direct ATA request to a hard drive (0x25, READ DMA EXT), to disobey max sector count (long story), and to bypass all possible OS caches, buffers, reorderings et al.
HDIO_DRIVE_TASKFILE IOCTL is no longer available due to libata.
I accomplished the goal with a SG_IO IOCTL with ATA pass-through (SG_ATA_16). Works perfectly except one problem: I can read a maximum of 8192 sectors in one command. I need to read a full of 32767 sectors.
- max_hw_sectors_kb is 32767, so the drive supports this much transfer
- max_sectors_kb was low, yet I brought it up to 32767 sectors, to no avail
- scheduler is set to noop, no change.
- Tried gather buffer (
iovec_count>0, properly setiovecsto consecutive buffer slices), no change.
Environment: Ubuntu 16.04/16.10/17.04 with standard kernels, SATA drive connected to standard AHCI interface on Intel chipset.
No matter what I do, starting with 8193 sectors, IOCTL bails out with "Invalid argument" error.
Where to look? What else can cause a 4MB data transfer cap?