I'm writing an initramfs-script and want to detect usb-sticks as fast as possible.When I insert an usb 2.0 stick, the detection of idVendor, idProduct and USB class happens within 100 ms. But the scsi subsystem does not attach until about 1 s has passed and it takes another 500 ms before the partition is fully recognized.I assume that the driver needs to read the partition table in order to detect partitions. Why does it take so long? I don't expect the urb send/recev time to be that long or the access time of the flash to take so much time.I've tried 5 sticks from different vendors and the result is about the same. [ 5731.097540] usb 2-1.2: new high-speed USB device number 7 using ehci-pci[ 5731.195360] usb 2-1.2: New USB device found, idVendor=0951, idProduct=1643[ 5731.195368] usb 2-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3[ 5731.195372] usb 2-1.2: Product: DataTraveler G3[ 5731.195376] usb 2-1.2: Manufacturer: Kingston[ 5731.195379] usb 2-1.2: SerialNumber: 001CC0EC32BCBBB04712022C[ 5731.196942] usb-storage 2-1.2:1.0: USB Mass Storage device detected[ 5731.197193] scsi host9: usb-storage 2-1.2:1.0[ 5732.268389] scsi 9:0:0:0: Direct-Access     Kingston DataTraveler G3  PMAP PQ: 0 ANSI: 0 CCS[ 5732.268995] sd 9:0:0:0: Attached scsi generic sg2 type 0[ 5732.883939] sd 9:0:0:0: [sdb] 7595520 512-byte logical blocks: (3.88 GB/3.62 GiB)[ 5732.884565] sd 9:0:0:0: [sdb] Write Protect is off[ 5732.884568] sd 9:0:0:0: [sdb] Mode Sense: 23 00 00 00[ 5732.885178] sd 9:0:0:0: [sdb] No Caching mode page found[ 5732.885181] sd 9:0:0:0: [sdb] Assuming drive cache: write through[ 5732.903834]  sdb: sdb1[ 5732.906812] sd 9:0:0:0: [sdb] Attached SCSI removable diskEditSo I've found the delay_use module parameter that by default is set to 1 second, which explains the delay I'm seeing. But I'm wondering if someone can provide context as to why that parameter is needed? A comment suggested that for older usb sticks, delay_use might need to be set to as much as 5 seconds. What is it inside the usb stick that takes so much time; firmware initialization; reads from the flash? I find it hard to belive that we need delays as long as 1 second or more when the latency for accessing flash is in the order of tens of microseconds.I realize that this might be slightly off-topic for this channel, if so, I'll go to electronics.stackexchange.com
Why does it take so long to detect an usb stick?
linux;linux kernel;usb;usb drive;scsi
You can change the timeout by writing to /sys/module/usb_storage/parameters/delay_use.For older usb disks, a settle delay of 5 seconds or even more may be needed (and 5 was the default until it was reduced to 1 second in 2010), presumably because the controller is starved of power while the disk motors are initializing.  Or possibly because the internal SCSI firmware takes time to boot up before it's responsive (can you tell I'm just speculating here?).For modern solid-state storage, it's probably not needed at all, and many people set it to 0. Unfortunately, it's a global parameter that applies to all devices, so if you have any slow devices at all, you have to endure the delay for every mass-storage USB device you use.  It would be nice if it could be set per-device by udev, but that's not the case.