loop-aes and dm-crypt are both used by partman-crypto for linux hard encryption, but dm-crypt needs no patch to kernel and is more supported. dm-crypt keeps it's key on first hard block, is it a security issue which make loop-aes better? which do you offer more? thanks
That's not entirely correct.
If you use dm-crypt by itself (without LUKS) then no key information is stored on the disk, and the key is re-derived every time you enter the passphrase. This isn't really any different from loop-aes.
Using dm-crypt With LUKS, your passphrase is used to encrypt the master key, and the encrypted master key is then written to the disk in a key slot. Since there are multiple key slots, the master key can be encrypted for different users, each with a unique passphrase.
Since the master key is encrypted before being written to the disk, you would still need to break the encryption on the key in order to use it.
Personally, I don't consider this a significant risk, and use dm-crypt with LUKS on all of the systems I am responsible for.
You can read more about dm-crypt (with and without LUKS) in the cryptsetup FAQ -