dd if=/dev/urandom of=/dev/sda unstable bitrate

[zhak]

I'm learning disk encryption, LUKS, and stuff atm, so I executed dd if=/dev/urandom of=/dev/sda. What I observe is bitrate instability: most of the time it is ~8MiB/s on my notebook, but it periodically drops to ~25KiB/s for a second or two every 200 MB or so. I wonder why is it happening? Does it tell me about some malfunction (problems with SSD or anything) or misconfiguration, or anything else?

[frostschutz]

urandom is way too slow to fill entire hard disks

if pseudorandom is good enough: shred -v -n 1 /dev/destroy

if encrypted random is good enough: cryptsetup plainOpen /dev/destroy cryptdestroy ; shred -v -n 1 /dev/mapper/cryptdestroy ; cryptsetup close cryptdestroy

even without AES-NI, crypto should be order of magnitude faster than urandom

In case of SSD, if you're planning to use TRIM, it's pointless in the first place. After TRIM it will just be zero again, so you can just as well trim the entire thing instead of overwriting: blkdiscard /dev/destroy


If you wish to stick to dd no matter what, always use bs=1M - dd uses only 512 byte blocksize by default which is slow for sheer read()/write() syscall overhead.

[Roman_Gruber]

shred is fast

urandom => depends on the hardware. sigh

why do you want to use shred in teh first place? one pass with shred is more than enough, with one bit pattern, thats it.

an ordinary user will be unable to recover data an anyone else use a gun on our heard, or has the money to recover it. physics sigh.


question @ frostschutz => do you use trim with ssd => lvm container -> luks -> root ext4.
I am still not sure if trim is worth on my setup. partition was aligned according to gpt partition table tool.

[frostschutz]

[Roman_Gruber]

Sorry to hijack a topic:

Wikipedia: https://en.wikipedia.org/wiki/Trim_%28computing%29

[frostschutz]

Huh?

I prefer fstrim (trim weekly or monthly) over discard (trim instantly) flag.

trim is supposed to keep the SSD happy in the long term.

discard (trim instantly) is a short term measure and if you ever have an accident, you won't get your data back.

(with fstrim I can disable the cron job and then photorec or whatever)

[Roman_Gruber]

cool thanks.

made it clear. Basically way a) and way b) to do the same stuff.